27Docker for Testing

27.1 Container vs VM

27.2 Core vocabulary

• Image — read-only template (filesystem + metadata). Built from a Dockerfile.
• Container — running instance of an image.
• Dockerfile — declarative recipe to build an image.
• Registry — where images are stored/shared (Docker Hub, GHCR, ECR).
• Layer — each Dockerfile step creates a cached layer. Order matters for cache efficiency.

27.3 Basic commands

docker pull mcr.microsoft.com/playwright:v1.50.0-jammy
docker images                        # list local
docker ps                            # running containers
docker ps -a                         # all containers
docker run -it --rm ubuntu bash      # ephemeral interactive
docker run -d --name mydb -p 5432:5432 -e POSTGRES_PASSWORD=secret postgres
docker logs mydb
docker exec -it mydb psql -U postgres
docker stop mydb && docker rm mydb
docker build -t my-tests .
docker push myregistry/my-tests:1.0

27.4 Dockerfile for a Playwright project

FROM mcr.microsoft.com/playwright:v1.50.0-jammy
WORKDIR /app

# Copy lockfile first → layer cache reuses when only code changes
COPY package*.json ./
RUN npm ci

# Copy source
COPY . .

# Default command
CMD ["npx", "playwright", "test"]

27.5 docker-compose — app + DB + tests

version: '3.8'
services:
  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret
      POSTGRES_DB: app_test
    ports: ["5432:5432"]
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 2s

  app:
    build: ./app
    depends_on:
      db: { condition: service_healthy }
    environment:
      DATABASE_URL: postgres://postgres:secret@db:5432/app_test
    ports: ["3000:3000"]

  tests:
    image: mcr.microsoft.com/playwright:v1.50.0-jammy
    depends_on: [app]
    environment:
      BASE_URL: http://app:3000
    volumes:
      - .:/work
    working_dir: /work
    command: npx playwright test

docker compose up --abort-on-container-exit --exit-code-from tests

27.6 Testcontainers (Java + others)

Library that spins up real services (Postgres, Kafka, Selenium Grid) from inside a JUnit test. Each test gets a fresh DB; no shared state.

@Testcontainers
class UserRepoTest {
  @Container static PostgreSQLContainer<?> pg = new PostgreSQLContainer<>("postgres:16");

  @Test
  void createsUser() throws Exception {
    DataSource ds = createDataSource(pg.getJdbcUrl(), pg.getUsername(), pg.getPassword());
    /* run migrations, exercise repo, assert */
  }
}

Available containers: Postgres, MySQL, Mongo, Redis, RabbitMQ, Kafka, Localstack (AWS), Selenium Standalone, Playwright (via GenericContainer).

27.7 Multi-stage builds

FROM node:20 AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM mcr.microsoft.com/playwright:v1.50.0-jammy
WORKDIR /app
COPY --from=build /app/dist ./dist
COPY --from=build /app/node_modules ./node_modules
COPY tests ./tests
COPY playwright.config.ts ./
CMD ["npx", "playwright", "test"]

Final image is smaller — no build tools, source maps, dev deps.

27.8 Common pitfalls

• Wrong base image — using node:20-alpine for Playwright → no Chrome dependencies, install fails. Use the official Playwright image.
• Layer cache busting — copying everything before npm ci means every code change invalidates the install layer. Copy lockfile first.
• Network between containers — inside docker-compose, use service names (db, app) not localhost.
• Permission issues — official Playwright image runs as pwuser; volume mounts may fail on chown. Run USER root or adjust ownership.
• Shared memory — Chrome needs more than the default 64 MB /dev/shm in containers. Set --shm-size=1gb or use --ipc=host.
• Disk bloat — old layers accumulate. docker system prune -a weekly.

27.9 docker for CI

Most CI runners (GitHub Actions, GitLab CI, Jenkins) support running steps inside a container. Pin the version of the Playwright image to match local — keeps trace/screenshot rendering identical and removes "passes locally fails CI" caused by image drift.