docs: update documentation with CI image info

README.md

@@ -373,6 +373,31 @@ The application uses Gitea Actions for continuous integration and deployment:
    - Docker image building and testing
    - Registry push and deployment
 
+4. **CI Image Builder** (`.gitea/workflows/ci-image-builder.yml`): Runs when dependencies change
+   - Automatically builds CI runner image with all dependencies pre-installed
+   - Triggered by changes to `package.json`, `package-lock.json`, or `Dockerfile.ci`
+
+### CI Runner Image
+
+To avoid installing dependencies on every workflow run, we use a pre-built CI runner image:
+
+**Dockerfile.ci** - Contains:
+- Node.js 20 Alpine
+- All npm dependencies pre-installed
+- Prisma client pre-generated
+- System tools (bash, git, python, make, g++)
+
+**Automatic Rebuilding:**
+- The CI image is automatically rebuilt when `package.json` or `package-lock.json` changes
+- Images are tagged with commit SHA for traceability
+- Falls back to `node:20-alpine` if custom image is not available
+
+**Manual Build:**
+```bash
+docker build -f Dockerfile.ci -t euchre-camp-ci:latest .
+docker run --rm euchre-camp-ci:latest node --version
+```
+
 ### Database Strategy for CI
 - **CI Tests**: SQLite database (fast, no server required)
 - **Production**: PostgreSQL (production-like environment)
@@ -385,6 +410,9 @@ npm run test:run
 
 # Run acceptance tests with SQLite
 DATABASE_PROVIDER=sqlite DATABASE_URL=file:./prisma/ci.db npm run test:acceptance
+
+# Run tests in CI container (if built)
+docker run --rm -v $(pwd):/app -w /app euchre-camp-ci:latest npm run test:run
 ```
 
 ## Docker Deployment