Add Docker Compose setup with PostgreSQL 16 and Spring Boot app

Multi-stage Dockerfile for the backend (build + runtime).
Compose defines db (postgres:16-alpine with healthcheck) and app services.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Dockerfile

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+FROM eclipse-temurin:21-jdk-alpine AS build
+WORKDIR /app
+COPY mvnw pom.xml ./
+COPY .mvn .mvn
+RUN ./mvnw dependency:go-offline -B
+COPY src src
+RUN ./mvnw package -DskipTests -B
+
+FROM eclipse-temurin:21-jre-alpine
+WORKDIR /app
+COPY --from=build /app/target/*.jar app.jar
+EXPOSE 8080
+ENTRYPOINT ["java", "-jar", "app.jar"]

docker-compose.yml

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+services:
+  db:
+    image: postgres:16-alpine
+    container_name: mealprep-db
+    environment:
+      POSTGRES_DB: mealprep
+      POSTGRES_USER: mealprep
+      POSTGRES_PASSWORD: mealprep
+    ports:
+      - "5432:5432"
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U mealprep"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  app:
+    build:
+      context: ./backend
+      dockerfile: Dockerfile
+    container_name: mealprep-app
+    ports:
+      - "8080:8080"
+    environment:
+      SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/mealprep
+      SPRING_DATASOURCE_USERNAME: mealprep
+      SPRING_DATASOURCE_PASSWORD: mealprep
+      SPRING_PROFILES_ACTIVE: docker
+    depends_on:
+      db:
+        condition: service_healthy
+
+volumes:
+  pgdata: