♻️ Move Dockerfile and rootfs to repo root per AI.md ♻️

- Dockerfile: moved from docker/Dockerfile; COPY paths now rootfs/ and Dockerfile
- rootfs/usr/local/bin/entrypoint.sh, healthcheck.sh: moved from docker/rootfs/
- rootfs/usr/local/share/prosody-plugins/: moved from docker/rootfs/ (mod_auth_openfire_hybrid, mod_user_api)
- docker/compose.yml: removed; the example now lives in the README compose section
- .github/workflows/ci.yml, release.yml: build file is now Dockerfile
- .github/CODEOWNERS: drop the docker/ entry
- .gitignore, .dockerignore: drop docker/volumes/
- README.md: compose section replaced with the current example

.github/CODEOWNERS

@@ -3,6 +3,3 @@
 
 # Workflow changes require maintainer sign-off
 .github/ @casjaysdevdocker
-
-# Docker assets
-docker/ @casjaysdevdocker