Add Docker publish workflow for automated image builds

[Thisen-Ekanayake]

Summary

• Adds .github/workflows/docker-publish.yml to automatically build and push
  the Docker image to GitHub Container Registry (GHCR) on every push to main/master
  and on semver tags (v*.*.*)
• Uses the built-in GITHUB_TOKEN — no external secrets or Docker Hub account required
• GitHub Actions layer caching (type=gha) keeps rebuilds fast

Image tags produced

Event	Tag
Push to main	ghcr.io/<owner>/openflowkit:main
Push tag v1.2.3	ghcr.io/<owner>/openflowkit:1.2.3 and :1.2
Any push	ghcr.io/<owner>/openflowkit:sha-<short-sha>

Test plan

• Workflow triggered on push to main and image built successfully
• Image pulled and deployed to Railway — app served correctly at the public URL

[Vrun-design]

Thanks for the contribution! Clean — merging.

[Thisen-Ekanayake]

You’re welcome! And thank you for building and maintaining this project. it’s really useful.