Docker Support

Concord supports running Docker images within a process flow.

• Usage
• Environment Variables
• Docker Options
  • Add Host Option
• Capturing the Output
• Custom Images
• Limitations

Usage

Short syntax:

flows:
  default:
    - docker: library/alpine
      cmd: echo '${greeting}'

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

Using the task syntax:

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

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

The above invocations are 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);
• stdout and stderr - optional, string. Name of variables to save the stdout and stderr of the container;
• 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:

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

To run multiple commands multiline YAML strings can be used:

- docker: 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:
    - docker: 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:
    - docker: 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:
  - docker: 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 parameters can be used to capture the output of commands running in the Docker container:

- docker: library/alpine
  cmd: echo "Hello, Concord!"
  stdout: myOut

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

In the example above the output (stdout) of the command running in the container is not printed out into the log, but instead saved as myOut variable.

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

- docker: library/alpine
  cmd: echo "Hello, ${name}" && (>&2 echo "STDERR WORKS")
  stderr: myErr

- log: "Errors: ${myErr}"

In the example above the errors (stderr) of the command running in the container is not printed out into the log, but instead saved as myErr 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.