When using dynamic port mapping or multiple services, it’s not always obvious which host port maps to which container port. docker compose port tells you exactly. Basic usage # Which host port maps to container port 80 on the web service? docker compose port web 80 # Output: 0.0.0.0:8080 Why dynamic ports matter When you let Docker assign ports automatically, the host port changes on every docker compose up: services: web: image: nginx ports: - "80" # Dynamic host port, container port 80 docker compose port web 80 # Output: 0.0.0.0:55432 (assigned dynamically) This is common in CI or when running multiple instances of the same project. ...

A sidecar is a helper container that runs alongside your main application, adding capabilities without modifying the application itself. Compose has specific features to make sidecars work seamlessly. Shared network namespace with network_mode Use network_mode: service:<name> to share the network stack with another container, they share the same IP address and can communicate over localhost: services: app: image: myapp # No ports needed — proxy handles public traffic proxy: image: nginx network_mode: service:app ports: - "443:443" volumes: - ./nginx.conf:/etc/nginx/nginx.conf:ro - ./certs:/etc/nginx/certs:ro Since both containers share the same network namespace, nginx can proxy to localhost:3000 without any DNS resolution. The app doesn’t need to know about TLS at all, the proxy sidecar handles it. ...

Build args and environment variables both pass values to your containers, but they work at different times and serve different purposes. Mixing them up is a common source of confusion. Build args: build-time only Build args are available during docker build and are not present in the running container: services: app: build: context: . args: NODE_VERSION: "20" APP_VERSION: "2.1.0" In the Dockerfile, they’re consumed with ARG: ...

Multi-stage Dockerfiles let you define multiple build stages. With the target option in Compose, you can choose which stage to build — giving you different images from the same Dockerfile. A multi-stage Dockerfile # Stage 1: dependencies FROM node:20-slim AS deps WORKDIR /app COPY package*.json ./ RUN npm ci # Stage 2: development (with dev dependencies and tools) FROM deps AS dev RUN npm install --include=dev COPY . . CMD ["npm", "run", "dev"] # Stage 3: build FROM deps AS build COPY . . RUN npm run build # Stage 4: production (minimal) FROM node:20-slim AS production WORKDIR /app COPY --from=build /app/dist ./dist COPY --from=deps /app/node_modules ./node_modules CMD ["node", "dist/index.js"] Targeting stages in Compose Use target to pick which stage to build: ...

When you run docker compose down or docker compose stop, Compose sends a signal to your containers. Understanding which signal is sent and how your application handles it is key to graceful shutdowns. Default behavior By default, Compose sends SIGTERM to the main process (PID 1), waits 10 seconds, then sends SIGKILL: services: app: image: myapp # Default: SIGTERM, 10s grace period, then SIGKILL Changing the stop signal Some applications expect a different signal. Nginx, for example, uses SIGQUIT for graceful shutdown: ...