Replace runtime mvn spring-boot:run with a proper multi-stage build:
- Stage 1 (builder): compiles JAR with BuildKit cache mount for ~/.m2
- Stage 2 (runtime): eclipse-temurin:21-jre with only the JAR
Removes the backend source volume mount and maven_cache named volume.
Deploy with: docker compose up -d --build
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>