Add automatic Docker builds on main branch

- docker-dev: Builds and pushes :dev tag on every main push
- docker-release: Builds :latest and :vX.Y.Z on tags
- docker-verify: Dry-run build on PRs

Updated README with Docker tag documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -156,7 +156,10 @@ git tag v1.0.0
 git push origin v1.0.0
 ```
 
-**Docker Images:** `ghcr.io/barryw/paperlessmcp:latest`
+**Docker Images:**
+- `ghcr.io/barryw/paperlessmcp:latest` — Latest release
+- `ghcr.io/barryw/paperlessmcp:dev` — Latest main branch
+- `ghcr.io/barryw/paperlessmcp:vX.Y.Z` — Specific version
 
 ### Project Structure