Fix Dockerfile ports, add Docker Compose, and update README

[ddulic]

Summary

• Fix SUPERNOTE_PORT env var and EXPOSE directive in Dockerfile from 8080 to 8000, aligning with the default port change in cdb725e
• Fix all port references in README (8080 → 8000)
• Fix broken Server Documentation link (was pointing to allenporter/supernote fork, now uses a relative path)
• Add docker-compose.yml with pre-built GHCR image (ghcr.io/allenporter/supernote), both ports, named volume, and annotated env vars
• Expand README Docker section with ghcr.io pull instructions and link to the new compose file

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

Thanks for integrating Codecov - We've got you covered ☂️