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

deploy/docker-compose.yml

@@ -0,0 +1,34 @@
+# Deploy descriptor for the 15-puzzle container on the host Docker daemon.
+# Driven by .gitea/workflows/ci.yaml (`docker compose up -d --build`); env
+# values are interpolated from Gitea Actions secrets/variables exported by the
+# deploy job. Joins the external `edge` network so the host reverse proxy can
+# route to `15-puzzle:8080`.
+name: 15-puzzle
+
+services:
+  app:
+    container_name: 15-puzzle
+    image: 15-puzzle:latest
+    build:
+      context: ..
+      dockerfile: Dockerfile
+    restart: unless-stopped
+    environment:
+      BOT_TOKEN: ${BOT_TOKEN:?set BOT_TOKEN}
+      ACCESS_CODE: ${ACCESS_CODE:?set ACCESS_CODE}
+      PROJECT_LINK: ${PROJECT_LINK:?set PROJECT_LINK}
+      DATA_FILE: ${DATA_FILE:-/data/data.json}
+      SERVER_PORT: ${SERVER_PORT:-8080}
+      CONTEXT_ROOT: ${CONTEXT_ROOT:-/}
+    volumes:
+      - data:/data
+    networks:
+      - edge
+
+volumes:
+  data:
+    name: 15-puzzle-data
+
+networks:
+  edge:
+    external: true