feat(01-03): create Dockerfile for containerization

- Add multi-stage Dockerfile (JDK build, JRE runtime)
- Use Eclipse Temurin 21 base images
- Configure health check on /health endpoint
- Add .dockerignore for efficient builds
- Environment variables for config override

.dockerignore

@@ -0,0 +1,30 @@
+# Build artifacts
+out/
+.bsp/
+
+# Git
+.git/
+.gitignore
+
+# IDE
+.idea/
+*.iml
+.vscode/
+
+# Reference implementation
+golem15-wintercms-starter/
+
+# Documentation
+*.md
+!README.md
+
+# Planning files
+.planning/
+
+# Infra (not needed in app container)
+infra/
+
+# Local environment
+.env
+.env.local
+*.log