🐳 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>

Dockerfile

@@ -1,57 +0,0 @@
-# DanceLessonsCoach Docker Image
-# Multi-stage build for production deployment
-
-# Stage 1: Build binary
-FROM golang:1.26.1-alpine AS builder
-
-WORKDIR /app
-
-# Copy go mod files
-COPY go.mod go.sum ./
-RUN go mod download
-
-# Copy source code
-COPY . ./
-
-# Install swag and generate Swagger docs only if they don't exist
-RUN if [ ! -f pkg/server/docs/swagger.json ]; then \
-    echo "📝 Generating Swagger documentation..." && \
-    go install github.com/swaggo/swag/cmd/swag@latest && \
-    cd pkg/server && go generate && \
-    echo "✅ Swagger documentation generated"; \
-    else \
-    echo "✅ Swagger documentation already exists, skipping swag installation and generation"; \
-    fi
-
-# Build binary
-RUN CGO_ENABLED=0 GOOS=linux go build -o /dance-lessons-coach ./cmd/server
-
-# Stage 2: Final image
-FROM alpine:3.18
-
-WORKDIR /app
-
-# Install dependencies
-RUN apk add --no-cache ca-certificates tzdata
-
-# Copy binary from builder
-COPY --from=builder /dance-lessons-coach /app/dance-lessons-coach
-
-# Copy configuration
-COPY config.yaml /app/config.yaml
-
-# Set permissions
-RUN chmod +x /app/dance-lessons-coach
-
-# Set timezone
-ENV TZ=UTC
-
-# Expose port
-EXPOSE 8080
-
-# Health check
-HEALTHCHECK --interval=30s --timeout=3s \
-    CMD wget -q --spider http://localhost:8080/api/health || exit 1
-
-# Entry point
-ENTRYPOINT ["/app/dance-lessons-coach"]