Container definitions

Note

This page reflects the options available in the most recent version of batect.

Each container definition is made up of:

image

Image name (in standard Docker image reference format) to use for this container. One of image or build_directory is required.

The image can be overridden when running a task with --override-image.

Tip

It is highly recommended that you specify a specific image version, and not use latest, to ensure that the same image is used everywhere. For example, use alpine:3.7, not alpine or alpine:latest.

build_directory

Path (relative to the configuration file's directory) to a directory containing a Dockerfile to build and use as an image for this container. One of image or build_directory is required.

On Windows, build_directory can use either Windows-style (path\to\thing) or Unix-style (path/to/thing) paths, but for compatibility with users running on other operating systems, using Unix-style paths is recommended.

The image can be overridden when running a task with --override-image.

build_args

List of build args (in name: value format) to use when building the image in build_directory. Values can be expressions.

Each build arg must be defined in the Dockerfile with an ARG instruction otherwise the value provided will have no effect.

Warning

Use caution when using build args for secret values. Build arg values can be revealed by anyone with a copy of the image with the docker history command.

dockerfile

Dockerfile (relative to build_directory) to use when building the image in build_directory. Defaults to Dockerfile if not set.

The Dockerfile must be within build_directory.

dockerfile must always be specified with Unix-style (path/to/thing) paths, even when running on Windows.

command

Command to run when the container starts.

If not provided, the default command for the image will be run.

Both of these can be overridden for an individual task by specifying a command at the task level.

Note

Keep in mind that this command is passed to the image's ENTRYPOINT, just like it would when using docker run <image> <command> directly.

This means that if the entrypoint is not set or is not a shell, standard shell syntax features like $MY_ENVIRONMENT_VARIABLE and && might not work.

See the Docker docs for CMD and ENTRYPOINT for more details.

If you would like to use shell syntax features in your command, you have four options:

  1. Create a shell script and invoke that instead of specifying the command directly.

  2. Wrap your command in a shell invocation.

    For example, if your command is echo hello && echo world, set command to sh -c 'echo hello && echo world'.

  3. Set the entrypoint in the image to a shell. For example:

    ENTRYPOINT ["/bin/sh", "-c"]
    

  4. Set the entrypoint for the container to a shell. For example:

    containers:
      container-1:
        command: "'echo hello && echo world'" # Single quotes so that whole command is treated as a single argument when passed to sh, double quotes so that YAML preserves the single quotes
        entrypoint: /bin/sh -c
    

Note that for both options 3 and 4, you must quote the command so that it is passed to sh -c as a single argument (we want the final command line to be sh -c 'echo hello && echo world', not sh -c echo hello && echo world).

entrypoint

Entrypoint to use to run the command.

If not provided, the default entrypoint for the container will be used.

Both of these can be overridden for an individual task by specifying an entrypoint at the task level.

See the Docker docs for CMD and ENTRYPOINT for more information on how the entrypoint is used. batect will always convert the entrypoint provided here to the exec form when passed to Docker.

environment

List of environment variables (in name: value format) for the container. Values can be expressions.

TERM

The TERM environment variable, if set on the host, is always automatically passed through to the container. This ensures that features such as coloured output continue to work correctly inside the container.

Proxy-related environment variables, if set on the host, are passed through to the container at build and run time, but are not used for image pulls.

If a proxy-related environment variable is defined on the container's configuration, it takes precedence over the host-provided value.

See this page for more information on using batect with proxies.

working_directory

Working directory to start the container in.

If not provided, the default working directory for the image will be used.

Both of these can be overridden for an individual task by specifying a working_directory at the task level.

volumes

List of volume mounts to create for the container.

Both local mounts (mounting a directory on the host into a container) and cache mounts are supported:

Local mounts

Two formats are supported:

  • local:container or local:container:options format

  • An expanded format:

    containers:
      my-container:
        ...
        volumes:
          # This is equivalent to .:/code:cached
          - local: .
            container: /code
            options: cached
    

In both formats, the following fields are supported:

  • local: path to the local file or directory to mount. Can be an expression when using the expanded format. Required.

    Relative paths will be resolved relative to the configuration file's directory.

    On Windows, the local path can use either Windows-style (path\to\thing) or Unix-style (path/to/thing) paths, but for compatibility with users running on other operating systems, using Unix-style paths is recommended.

  • container: path to mount the local file or directory at inside the container. Required.

  • options: standard Docker mount options (such as ro for read-only). Optional.

Using options: cached may improve performance when running on macOS hosts - see this page for further explanation.

Cache mounts

Cache mounts provide persistence between task runs without the performance overhead of mounting a directory from the host into the container.

They are perfect for directories such as node_modules which contain downloaded dependencies that can safely be reused for each task run.

The format for a cache mount is:

containers:
  my-container:
    ...
    volumes:
      - type: cache
        name: node-modules
        container: /code/node_modules

The following fields are supported:

  • type: must be set to cache. Required.
  • name: name of the cache, must be a valid Docker volume name. The same name can be used to share a cache between multiple containers. Required.
  • container: path to mount the cache directory at inside the container. Required.
  • options: standard Docker mount options (such as ro for read-only). Optional.

devices

List of device mounts to create for the container.

Two formats are supported:

  • local:container or local:container:options format

  • An expanded format:

    containers:
      my-container:
        ...
        devices:
        # This is equivalent to /dev/sda:/dev/disk:r
        - local: /dev/sda
          container: /dev/disk
          options: r
    

Note that the local device mounts will be different for Windows and Unix-like hosts. See the Docker guide for adding host devices to containers for more information.

ports

List of ports to make available to the host machine.

Three formats are supported:

  • local:container format

    For example, 1234:5678 will make port 5678 inside the container available on the host machine at port 1234.

  • local_from-local_to:container_from:container-to format

    For example, 1000-1001:2025-2026 will make port 2025 inside the container available on the host machine at port 1000, and port 2026 inside the container available on the host machine at port 1001.

  • An expanded format:

    containers:
      my-container:
        ...
        ports:
          # This is equivalent to 1234:5678
          - local: 1234
            container: 5678
          # This is equivalent to 1000-1001:2025-2026
          - local: 1000-1001
            container: 2025-2026
    

Only TCP ports are supported.

Tip

Exposing ports is only required if you need to access the container from the host machine.

Any container started as part of a task will be able to access any port on any other container at the address container_name:container_port, even if that port is not listed in ports.

For example, if a process running in the http-server container listens on port 2000, any other container in the task can access that at http-server:2000 without port 2000 being listed in ports (or an EXPOSE Dockerfile instruction).

dependencies

List of other containers that should be started and healthy before starting this container.

If a dependency's image does not contain a health check, then as soon as it has started, it is considered to be healthy.

See this page for more information on how to ensure dependencies are ready before starting containers that depend on them.

health_check

Overrides health check configuration specified in the image or Dockerfile:

  • command The command to run to check the health of the container. If this command exits with code 0, the container is considered healthy, otherwise the container is considered unhealthy. If not provided, the default command specified in the image or Dockerfile is used.

  • retries The number of times to perform the health check before considering the container unhealthy.

  • interval The interval between runs of the health check. Accepts values such as 2s (two seconds) or 1m (one minute).

  • start_period The time to wait before failing health checks count against the retry count. The health check is still run during this period, and if the check succeeds, the container is immediately considered healthy. Accepts values such as 2s (two seconds) or 1m (one minute).

run_as_current_user

Run the container with the same UID and GID as the user running batect (rather than the user the Docker daemon runs as, which is root on Linux). This means that any files created by the container will be owned by the user running batect, rather than root.

This is really only useful on Linux. On macOS, the Docker daemon runs as the currently logged-in user and so any files created in the container are owned by that user, so this is less of an issue. However, for consistency, the same configuration changes are made on both Linux and macOS.

run_as_current_user has the following options:

  • enabled Defaults to false, set to true to enable 'run as current user' mode.

  • home_directory Directory to use as home directory for user inside container. Required if enabled is true, not allowed if enabled is not provided or set to false.

This directory is automatically created by batect with the correct owner and group.

Warning

If the directory given by home_directory already exists inside the image for this container, it is overwritten.

See this page for more information on the effects of this option and why it is necessary.

setup_commands

List of commands to run inside the container after it has become healthy but before dependent containers start.

  • command The command to run. Required.

    This command is run in a similar way to the container's command, so the same limitations apply to using shell syntax such as &&.

  • working_directory The working directory to use for the command.

    If no working directory is provided, working_directory is used if it is set, otherwise the image's default working directory is used. If this container is used as the task container and the task overrides the default working directory, that override is ignored when running setup commands.

The command will inherit the same environment variables as the container's command (including any specified on the task if this is the task container), runs as the same user and group as the container's command and inherits the same settings for privileged status and capabilities.

See the task lifecycle for more information on the effects of this option.

Tip

It is recommended that you try to include any setup work in your image's Dockerfile wherever possible (and not use setup commands), as setup commands must be run every time the container starts whereas commands included in your image's Dockerfile only run when the image needs to be built.

privileged

Set to true to run the container in privileged mode.

See also capabilities_to_add and capabilities_to_drop.

capabilities_to_add and capabilities_to_drop

List of capabilities to add or drop for the container.

This is equivalent to passing --cap-add or --cap-drop to docker run.

enable_init_process

Set to true to pass the --init flag when running the container. This creates the container with a simple PID 1 process to handle the responsibilities of the init system, which is required for some applications to behave correctly.

Read this article if you're interested in more information about the behaviour of different processes running as PID 1 and why this flag was introduced.

additional_hostnames

List of hostnames to associate with this container, in addition to the default hostname (the name of the container).

Examples

For more examples and real-world scenarios, take a look at the sample projects.

Minimal configuration with existing image

containers:
  build-env:
    image: openjdk:8u141-jdk

Running the container build-env will launch a container that uses the openjdk:8u141-jdk image.

If the image has not already been pulled, batect will pull it before starting the container.

Minimal configuration with Dockerfile

containers:
  build-env:
    build_directory: .batect/build-env

Running the container build-env will first build the Dockerfile in the .batect/build-env directory, then run the resulting image.

The Docker build cache is used during the build process, so if the image definition has not changed since the last build, the image will not be rebuilt, saving time.

Container with custom command

containers:
  build-env:
    image: ruby:2.4.3
    command: echo 'Hello world'

Running the container build-env will run the command echo 'Hello world', and not the default command specified in the ruby:2.4.3 image.

This command could, however, be overridden by specifying a command at the task level.

Container with environment variables

containers:
  build-env:
    image: ruby:2.4.3
    environment:
      COUNTRY: Australia
      SUPER_SECRET_VALUE: $SECRET_PASSWORD
      ANOTHER_SECRET_VALUE: ${SECRET_PASSWORD}
      OPTIMISATION_LEVEL: ${HOST_OPTIMISATION_LEVEL:-none}

Running the container build-env will launch a container that uses the ruby:2.4.3 image with the following environment variables:

  • The environment variable COUNTRY will have value Australia.

  • The environment variables SUPER_SECRET_VALUE and ANOTHER_SECRET_VALUE will have the value of the SECRET_PASSWORD environment variable on the host. (So, for example, if SECRET_PASSWORD is abc123 on the host, then SUPER_SECRET_VALUE will have the value abc123 in the container.)

    If SECRET_PASSWORD is not set on the host, batect will show an error message and not start the task.

  • The environment variable OPTIMISATION_LEVEL will have the value of the HOST_OPTIMISATION_LEVEL environment variable on the host.

    If HOST_OPTIMISATION_LEVEL is not set on the host, then OPTIMISATION_LEVEL will have the value none in the container.

These environment variables could be overridden (and added to) with environment at the task level.

Container with working directory

containers:
  build-env:
    image: ruby:2.4.3
    working_directory: /somewhere

Running the container build-env will launch a container that uses the ruby:2.4.3 image with the working directory set to /somewhere.

Container with volume mounts

containers:
  build-env:
    image: ruby:2.4.3
    volumes:
      - local: .
        container: /code
        options: cached

Running the container build-env will launch a container that uses the ruby:2.4.3 image, with the directory containing the batect configuration file mounted into the container at /code.

For example, if the batect configuration file is on the host at /home/alice/code/my-project/batect.yml, then /home/alice/code/my-project will be available inside the container at /code.

See this page for more information on why using cached volume mounts may be worthwhile.

Container with ports

containers:
  build-env:
    image: ruby:2.4.3
    ports:
      - local: 123
        container: 456

Running the container build-env will launch a container that uses the ruby:2.4.3 image, with the port 123 on the host mapped to port 456 inside the container. For example, this means that if a web server is listening on port 456 within the container, it can be accessed from the host at http://localhost:123.

The Dockerfile for the ruby:2.4.3 image does not need to contain an EXPOSE instruction for port 456.

Note that this does not affect how containers launched by batect as part of the same task access ports used by each other, just how they're exposed to the host. Any container started as part of a task will be able to access any port on any other container at the address container_name:container_port. For example, if a process running in another container wants to access the application running on port 456 in the build-env container, it would access it at build-env:456, not build-env:123.

Container with dependencies

containers:
  application:
    build_directory: .batect/application
    dependencies:
      - database

  database:
    build_directory: .batect/database

Running the container application will first run the database container and wait for it to become healthy before starting the application container.

Container that runs as the current user

containers:
  build-env:
    image: ruby:2.4.3
    run_as_current_user:
      enabled: true
      home_directory: /home/container-user

Running the container build-env will launch a container that uses the ruby:2.4.3 image with run as current user mode enabled.

Container that runs with Docker's default init process enabled

containers:
  build-env:
    image: node:10.10.0-alpine
    volumes:
      - local: .
        container: /code
        options: cached
    enable_init_process: true

Running the container build-env will launch a container that uses the node:10.10.0-alpine image with Docker's default init process as PID 1.

Container that runs a setup command after starting

containers:
  database:
    build_directory: .batect/database
    setup_commands:
      - command: ./apply-migrations.sh

  application:
    build_directory: .batect/application
    dependencies:
      - database

Running the container application will first build the images for both the database and application containers.

Once the image for database is ready, database will start and launch the command specified in the Dockerfile, then batect will wait for the container to report as healthy. Once database reports as healthy, it will run ./apply-migrations.sh and wait for it to finish before then starting application.