chore: add Docker CI compose, Gradle E2E task, and .dockerignore

Add infrastructure for running Playwright E2E tests in Docker and fix
Gradle lock conflicts between host and container builds.

Changes:
- Add docker-compose.ci.yml that starts postgres, backend, frontend,
  and a Playwright service for CI pipelines. Uses official
  mcr.microsoft.com/playwright:v1.60.0-noble image.
- Add backend-gradle-project named volume to docker-compose.yml so the
  container's .gradle/ directory is isolated from the host's. This
  prevents stale lock files from host Gradle builds (e.g. ./gradlew
  :backend:test) crashing the container's bootRun.
- Add .dockerignore excluding .gradle, .env, .git, frontend/node_modules,
  and backend/build from the Docker build context.
- Add frontendE2E Gradle task that runs npm run test:e2e:ci.

.dockerignore

@@ -0,0 +1,5 @@
+.gradle
+.env
+.git
+frontend/node_modules
+backend/build

build.gradle

@@ -15,6 +15,12 @@ tasks.register('frontendTest', Exec) {
     commandLine 'npm', 'run', 'test'
 }
 
+tasks.register('frontendE2E', Exec) {
+    description = 'Run Playwright E2E tests in Docker (CI mode)'
+    workingDir = file("${rootProject.projectDir}/frontend")
+    commandLine 'npm', 'run', 'test:e2e:ci'
+}
+
 tasks.named('check').configure {
     dependsOn frontendLint, frontendTest
 }

docker-compose.ci.yml

@@ -0,0 +1,63 @@
+services:
+  postgres:
+    image: postgres:16
+    container_name: bilhej-postgres-ci
+    environment:
+      POSTGRES_DB: ${POSTGRES_DB}
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  backend:
+    build:
+      dockerfile: docker/backend.Dockerfile
+      context: .
+    container_name: bilhej-backend-ci
+    environment:
+      SPRING_PROFILES_ACTIVE: docker
+      POSTGRES_DB: ${POSTGRES_DB}
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+      JWT_SECRET: ${JWT_SECRET}
+      STRIPE_SECRET_KEY: ${STRIPE_SECRET_KEY}
+      STRIPE_WEBHOOK_SECRET: ${STRIPE_WEBHOOK_SECRET}
+      STRIPE_PRICE_ID: ${STRIPE_PRICE_ID}
+    depends_on:
+      postgres:
+        condition: service_healthy
+    volumes:
+      - .:/app
+      - gradle-cache:/root/.gradle
+
+  frontend:
+    build:
+      dockerfile: docker/frontend.Dockerfile
+      context: .
+    container_name: bilhej-frontend-ci
+    depends_on:
+      - backend
+
+  playwright:
+    image: mcr.microsoft.com/playwright:v1.60.0-noble
+    container_name: bilhej-playwright
+    ipc: host
+    working_dir: /app
+    environment:
+      PLAYWRIGHT_BASE_URL: http://frontend:3000
+    volumes:
+      - ./frontend/package.json:/app/package.json
+      - ./frontend/package-lock.json:/app/package-lock.json
+      - ./frontend/playwright.config.ts:/app/playwright.config.ts
+      - ./frontend/e2e:/app/e2e
+      - ./frontend/node_modules:/app/node_modules
+    depends_on:
+      - frontend
+    command: >
+      sh -c "npm ci --ignore-scripts && npx playwright test --reporter=list"
+
+volumes:
+  gradle-cache:

docker-compose.yml

@@ -37,6 +37,7 @@ services:
         condition: service_healthy
     volumes:
       - .:/app
+      - backend-gradle-project:/app/.gradle
       - gradle-cache:/root/.gradle
 
   frontend:
@@ -56,3 +57,4 @@ services:
 volumes:
   pgdata:
   gradle-cache:
+  backend-gradle-project: