WIP [JENKINS-68955] Add inside() method to Container class

[tympanix]

Implements Container.inside() to execute a closure within an already-running container, mirroring Image.inside() behavior.

Changes

• WithContainerStep.java: Made Decorator class package-private (was private) to allow reuse by WithRunningContainerStep

• WithRunningContainerStep.java (NEW): A new step class for running build steps inside an already-running container

  • Takes containerId as required parameter
  • Validates container exists and is running before execution
  • Re-uses WithContainerStep.Decorator for docker exec wrapping
  • Registered as withRunningDockerContainer step

• Docker.groovy: Added Container.inside(Closure body) method

  • Delegates to withRunningDockerContainer step with the container's ID

• DockerDSLTest.java:

  • Added containerInside() test for the new functionality
  • Added containerInsideNonExistingContainer() test to validate error handling when using a non-existing container id

Usage

docker.image('httpd:2.4.62').withRun { c ->
    c.inside {
        sh 'cat /usr/local/apache2/conf/extra/httpd-userdir.conf'
    }
    // Container remains running here
}

Testing done

Added two test cases to validate:

• Executing the inside method on a container object (from withRun) yields an execution context in the already running container
• Executing the wintRunningDockerContainer with a non-existing container id yields an exception

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
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• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed