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

Dockerfile

@@ -0,0 +1,51 @@
+# SummerCMS Docker Build
+# Multi-stage build: JDK for building, JRE for runtime
+
+# Stage 1: Build
+FROM eclipse-temurin:21-jdk AS builder
+
+# Install Mill build tool
+# Using ./mill launcher pattern - download specific version
+WORKDIR /app
+
+# Copy mill launcher and version first for caching
+COPY .mill-version ./
+COPY mill ./
+
+# Copy build files
+COPY build.mill ./
+
+# Copy source code
+COPY summercms ./summercms
+
+# Build fat JAR using local mill launcher
+RUN chmod +x ./mill && ./mill summercms.assembly
+
+# Stage 2: Runtime
+FROM eclipse-temurin:21-jre
+
+WORKDIR /app
+
+# Install curl for health check
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends curl && \
+    rm -rf /var/lib/apt/lists/*
+
+# Copy the fat JAR from builder
+COPY --from=builder /app/out/summercms/assembly.dest/out.jar /app/summercms.jar
+
+# Health check using /health endpoint
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+    CMD curl -f http://localhost:8080/health || exit 1
+
+EXPOSE 8080
+
+# Environment variables for configuration override
+# These can be overridden at runtime with -e flags
+ENV DB_HOST=localhost
+ENV DB_PORT=5432
+ENV DB_NAME=summercms
+ENV DB_USER=summercms
+
+# Run the application
+ENTRYPOINT ["java", "-jar", "/app/summercms.jar"]