Support parameterized DockerCloud agent images

[hshibuya-dd]

Allow DockerCloud agent template images to reference Jenkins build parameters, for example:

registry.example.com/ci/build-agent:${IMAGE_TAG}

This lets a job use its default image tag for normal builds while selecting an isolated pull request image tag, such as pr-1234, for targeted validation.

Implementation:

• Resolve parameter values when a queue item enters the waiting state.
• Clone parameterized templates per queue item instead of mutating persisted DockerCloud configuration.
• Preserve the configured template's container settings, connector, pull strategy, timeouts, limits, retention strategy, and node properties.
• Assign each temporary template a unique label so concurrent builds do not share image selections.
• Remove temporary templates when queue items leave the queue.
• Cancel queue items whose temporary Docker image cannot be provisioned, so failed image pulls do not remain queued indefinitely.
• Ignore missing or empty parameter values rather than constructing an invalid image reference.
• Document parameterized image configuration in the README and image help text.

Tests:

• Verify image parameter expansion to a pull request tag.
• Verify missing and empty parameters leave the image unresolved.
• Verify cloned templates retain their configured settings while replacing the image and label.

Testing done

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
• Please describe what you did
• 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