Edit this page on GitHub

Home > docs > plugins v2 > Docker Support

Docker Support

Concord supports running Docker images within a process flow.

Usage

The docker task is called with standard runtime-v2 task call syntax.

flows:
  default:
    - task: docker
      in:
        image: library/alpine
        cmd: echo '${greeting}'
      out: dockerResult

configuration:
  arguments:
    greeting: "Hello, world!"

The above invocation is equivalent to running

docker pull library/alpine && \
docker run -i --rm \
-v /path/to/process/workDir:/workspace \
library/alpine \
echo 'Hello, world!'

Parameters

  • image - mandatory, string. Docker image to use;
  • cmd - optional, string. Command to run. If not specified, the image’s ENTRYPOINT is used;
  • env - optional, environment variables;
  • envFile - optional. Path to the file containing environment variables;
  • hosts - optional. Additional /etc/host entries;
  • forcePull - optional, boolean. If true Concord runs docker pull ${image} before starting the container. Default is true;
  • debug - optional, boolean. If true Concord prints out additional information into the log (the command line, parameters, etc);
  • logOutput - optional boolean. Sends container log output to Concord process logs. Default is true;
  • pullRetryCount - optional, number. Number of retries if docker pull fails. Default is 3;
  • pullRetryInterval - optional, number. Delay in milliseconds between docker pull retries. Default is 10000.

Note: The current process’ working directory is mounted as /workspace. Concord replaces the container’s WORKDIR with /workspace. Depending on your setup, you may need to change to a different working directory:

- task: docker
  in:
    image: library/alpine
    cmd: cd /usr/ && echo "I'm in $PWD"

To run multiple commands multiline YAML strings can be used:

- task: docker
  in:
    image: library/alpine
    cmd: |
      echo "First command"
      echo "Second command"
      echo "Third command"

Concord automatically removes the container when the command is complete.

Environment Variables

Additional environment variables can be specified using env parameter:

flows:
  default:
  - task: docker
    in:
      image: library/alpine
      cmd: echo $GREETING
      env:
        GREETING: "Hello, ${name}!"

configuration:
  arguments:
    name: "concord"

Environment variables can contain expressions: all values will be converted to strings.

A file containing environment variables can be used by specifying the envFile parameter:

flows:
  default:
  - task: docker
    in:
      image: library/alpine
      cmd: echo $GREETING
      envFile: "myEnvFile"

The path must be relative to the process’ working directory ${workDir}.

It is an equivalent of running docker run --env-file=myEnvFile.

Docker options

Add Host Option

Additional /etc/hosts lines can be specified using hosts parameter:

flows:
  default:
  - task: docker
    in:
      image: library/alpine
      cmd: echo '${greeting}'
      hosts:
        - foo:10.0.0.3
        - bar:10.7.3.21

configuration:
  arguments:
    greeting: "Hello, world!"

Capturing the Output

The stdout and stderr attributes of the task’s returned data can be used to capture the output of commands running in the Docker container:

- task: docker
  in:
    image: library/alpine
    cmd: echo "Hello, Concord!"
  out: dockerResult

- log: "Got the greeting: ${dockerResult.stdout.contains('Hello')}"

In the example above, the output (stdout) of the command running in the container is accessible in the returned object’s stdout attribute.

The stderr parameter can be used to capture the errors of commands running in the Docker container:

- task: docker
  in:
    image: library/alpine
    cmd: echo "Hello, ${name}" && (>&2 echo "STDERR WORKS")
  out: dockerResult

- log: "Errors: ${dockerResult.stderr}"

In the example above the errors (stderr) of the command running in the container is accessible in the returned object’s stderr variable.

Custom Images

Currently, there’s only one requirement for custom Docker images: all images must provide a standard POSIX shell as /bin/sh.

Limitations

Running containers as root user is not supported - all user containers are executed using the concord user equivalent to a run command like docker run -u concord ... myImage. The user is created automatically with UID 456.

As a result any operations in the docker container that require root access, such as installing packages, are not supported on Concord. If required, ensure that the relevant package installation and other tasks are performed as part of your initial container image build and published to the registry from which Concord retrieves the image.