Add image publishing job and size report - #78

Summary

• add a dedicated Jenkins pipeline for building and publishing a tagged backend image
• generate and archive a Markdown report containing the image size and identifiers
• add a reusable publishing script with release-ref and Docker-tag validation
• add a fake Docker implementation and tests for successful publishing, invalid branch refs, and failed builds
• run the safe publishing-script test in the existing Docker GitHub Actions workflow
• ignore the generated image report

Why

This provides a repeatable image-release job and makes the resulting image size visible as a Jenkins artifact. The publishing logic is kept in a standalone script so it can be tested without contacting Docker or the production registry.

Validation

• bash -n ci/publish-image.sh ci/tests/fake-docker ci/tests/publish-image-test.sh
• ./ci/tests/publish-image-test.sh
• parsed .github/workflows/docker.yml as YAML
• confirmed the test performs exactly one simulated build and push
• confirmed invalid branch refs are rejected before Docker is called
• confirmed a failed build prevents the simulated push

No real container image was pushed during local validation. The local environment uses Podman without Buildx Bake support, so a real Buildx build and Jenkins execution remain to be validated in CI.

First-time contribution note

This is my first time implementing this kind of CI and container-image publishing workflow. For transparency, I used AI for guidance, explanations, and review, but I typed the implementation myself and checked and rechecked every change to make sure I understood it.

I would particularly appreciate feedback on the Jenkins structure, registry authentication, release-tag validation, test coverage, and image-size reporting. If this approach is suitable, I would like to apply similar changes to the other backend repositories.