ci: publish container images

[rmkraus]

Summary

• Add a GitHub Actions workflow to build container images for the combined app, backend, and frontend Dockerfiles.
• Publish images to GHCR on pushes to main.
• Build images without pushing on pull requests for validation.
• Tag published images with sha- and latest on the default branch.

Images

• ghcr.io//-warehouse-assistant
• ghcr.io//-warehouse-assistant-backend
• ghcr.io//-warehouse-assistant-frontend

Test Plan

• Parsed .github/workflows/container-images.yml with PyYAML.
• Ran git diff --check.
• Built all three images locally with docker buildx:
  • Dockerfile
  • Dockerfile.backend
  • Dockerfile.frontend

Notes

This is the image-publishing prerequisite for a future Helm chart.

[rmkraus]

closing for now, we will revisit later.