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.
