Add Gitea Actions CI/CD and deploy descriptor

- .gitea/workflows/ci.yaml: test on PR/push to master, deploy on push to master
- deploy/docker-compose.yml: 15-puzzle container on the edge network with a persistent data volume
- Dockerfile: create /data owned by app for the DATA_FILE volume
- README: document deployment, required Actions secrets/variables, and persistence

README.md

@@ -37,6 +37,40 @@ Certain environment variables should be set for the server to start:
 | `CONTEXT_ROOT`     | Server requests URI root path, defaulting to `/` if not set. |
 | `STATIC_DIR`       | Directory where static files are located, defaulting to the current directory if not set. |
 
+## Deployment
+
+The repository ships a [Gitea Actions](https://docs.gitea.com/usage/actions/overview)
+workflow ([`.gitea/workflows/ci.yaml`](.gitea/workflows/ci.yaml)) that runs the
+server test suite on every pull request to `master`, and on every push to
+`master` additionally rebuilds the Docker image and (re)deploys the container on
+the host Docker daemon through [`deploy/docker-compose.yml`](deploy/docker-compose.yml).
+
+The deploy job runs in host mode: it builds the image locally (no registry) and
+recreates the `15-puzzle` container attached to the external `edge` Docker
+network, so a host reverse proxy can route to `15-puzzle:8080`.
+
+### Required Actions secrets and variables
+
+Configure these under the repository's *Settings → Actions*:
+
+| Kind     | Name           | Example / note |
+| -        | -              | -              |
+| Secret   | `BOT_TOKEN`    | Telegram Bot API token. |
+| Secret   | `ACCESS_CODE`  | Pin-code for the Statistics Screen. |
+| Variable | `PROJECT_LINK` | URL to the project's source code. |
+| Variable | `DATA_FILE`    | `/data/data.json` (lives on the persistent volume). |
+| Variable | `SERVER_PORT`  | `8080`. |
+| Variable | `CONTEXT_ROOT` | `/`. |
+
+`STATIC_DIR` is intentionally left unset: the static assets (HTML, `game.wasm`,
+`wasm_exec.js`) are baked into the image and served from the working directory.
+
+### Persistence
+
+Game data is stored on the named Docker volume `15-puzzle-data` mounted at
+`/data`, so player ratings survive container recreation. The image creates
+`/data` owned by the unprivileged `app` user so the volume is writable.
+
 ## Credits
 
 - [Ebitengine](https://github.com/hajimehoshi/ebiten) game engine by Hajime Hoshi.