Add transparent Docker fallback when CVMFS sandbox not found

[Copilot]

Summary

Updated Docker fallback implementation based on code review feedback.

Completed Tasks

• Understand current codebase structure and flow
• Modify run-linux.sh to detect when CVMFS path doesn't exist
• Add Docker fallback logic to start container when CVMFS path not found
• Ensure Docker container naming for persistence through cleanup
• Generate Docker image name from organization/platform/release parameters
• Execute user payload in Docker container when using fallback
• Address code review feedback (container naming, user mapping, error handling)
• Update documentation to explain Docker fallback feature
• Add explicit error handling for Docker operations
• Add explicit tests for Docker fallback functionality
• Update tests to use ghcr.io/eic/eic_cvmfs:nightly
• Remove workflow_dispatch restriction from reuse test
• Run tests on all branches
• Mount /cvmfs in Docker containers
• Remove redundant CVMFS test
• Remove diagnostic message step
• Use handlebar variables for image name
• All tasks completed

Latest Changes

• Removed diagnostic message step that instructed users to check logs
• Updated hard-coded image name to use handlebar variables for dynamic resolution

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support transparent running against ghcr.io</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.
When building containers we are not able to publish the container sandbox image to cvmfs (the preferred and most efficient approach to loading containers in CI). In those cases we only have the full container as pushed to ghcr.io. In order to test the repositories that use this action with containers on ghcr.io we want to be able to transparently fall through from cvmfs to ghcr.io.

Describe the solution you'd like
When a release:platform is not found under the respective directory on cvmfs, we should start a docker container that is used to exec the specified commands in. As in the case of cvmfs containers, the docker containers should continue to exist through to the post cleanup.

Describe alternatives you've considered
The alternative of requiring changes in all places that use this action is imposing a large amount of responsibility on the downstream users.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Support transparent running against ghcr.io #29

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[wdconinc]

@copilot Add explicit tests for the docker fall through functionality.

[Copilot]

@copilot Add explicit tests for the docker fall through functionality.

Added comprehensive test workflow in commit 0281436. The workflow includes:

1. Fallback Detection Test - Verifies Docker fallback is triggered when CVMFS path doesn't exist
2. Container Reuse Test - Validates that containers persist and are reused across multiple action invocations (manual workflow_dispatch only)
3. CVMFS Unchanged Test - Ensures existing Apptainer functionality still works correctly

The workflow supports both automatic testing (on PR/push for detection logic) and manual testing (workflow_dispatch with custom Docker images that exist on ghcr.io).

[Copilot]

Pull request overview

This PR adds transparent Docker fallback functionality that automatically uses Docker containers from ghcr.io when the requested container image is not found on CVMFS. This enables testing of containers that haven't been published to CVMFS yet without requiring workflow changes.

Changes:

• Modified run-linux.sh to detect when CVMFS sandbox path doesn't exist and fall back to Docker
• Added Docker container management logic with persistent containers and error handling
• Updated documentation to explain the Docker fallback feature

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
run-linux.sh	Implements Docker fallback detection, container creation/reuse logic, and payload execution in Docker containers when CVMFS path is not found
README.md	Documents the new Docker fallback feature, its capabilities, and how it works
.github/workflows/docker-fallback.yml	Adds comprehensive tests for Docker fallback functionality including detection and container reuse

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