build: optimize Dockerfiles for production

- Use BuildKit syntax 1.7 with cache mounts for apt/uv
- Switch from pip to uv for 10-100x faster installs (ADR-0014)
- Add OCI Image Spec labels for container metadata
- Add HEALTHCHECK directives for orchestration
- Add .dockerignore to reduce context size
- Update Makefile with buildx and lint target
- Add retry logic to ray-entrypoint.sh

Refs: ADR-0012 (uv), ADR-0014 (Docker best practices)

.dockerignore

@@ -0,0 +1,44 @@
+# Git
+.git
+.gitignore
+.gitea
+
+# Documentation
+*.md
+LICENSE
+docs/
+
+# IDE and editors
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# Python artifacts
+__pycache__/
+*.py[cod]
+*$py.class
+.pytest_cache/
+.venv/
+venv/
+.env
+*.egg-info/
+dist/
+build/
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Build logs
+*.log
+*.tmp
+
+# Local development
+Makefile
+.goreleaser.yml
+
+# Don't ignore these (explicitly include)
+!ray-serve/
+!dockerfiles/