🐳 feat: implement Docker multi-stage build with caching optimization

Added Docker build infrastructure:
- Multi-stage build (builder, cache, production)
- Dependency hashing for cache invalidation
- GNU tar support for cache compatibility
- Production and development Dockerfiles
- Docker Compose for local development

Build Optimization:
- Dependency-based cache keys
- Layer caching strategy
- Cross-platform compatibility
- Gitea Actions cache integration

Files Added:
- docker/Dockerfile.build - Build environment
- docker/Dockerfile.prod - Production image
- docker/Dockerfile.prod.template - Template-based generation
- docker-compose.yml - Development setup
- scripts/calculate-deps-hash.sh - Cache key calculation
- scripts/test-docker-cache.sh - Cache testing

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

docker-compose.yml

@@ -0,0 +1,47 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: dance-lessons-coach-postgres
+    environment:
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+      POSTGRES_DB: dance_lessons_coach
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+    networks:
+      - dance-lessons-coach-network
+    restart: unless-stopped
+
+  # Application service (for reference)
+  # app:
+  #   build: .
+  #   container_name: dance-lessons-coach-app
+  #   ports:
+  #     - "8080:8080"
+  #   environment:
+  #     - DLC_DATABASE_HOST=postgres
+  #     - DLC_DATABASE_PORT=5432
+  #     - DLC_DATABASE_USER=postgres
+  #     - DLC_DATABASE_PASSWORD=postgres
+  #     - DLC_DATABASE_NAME=dance_lessons_coach
+  #     - DLC_DATABASE_SSL_MODE=disable
+  #   depends_on:
+  #     postgres:
+  #       condition: service_healthy
+  #   restart: unless-stopped
+
+volumes:
+  postgres_data:
+    driver: local
+
+networks:
+  dance-lessons-coach-network:
+    name: dance-lessons-coach-network
+    driver: bridge