Add Docker deployment via Gitea Actions host-mode runner

Containerize the app (Dockerfile, docker-compose.yaml) and add a CI
deploy job that syncs the checkout to /srv/docker/maptool and runs
docker compose directly, using the runner's new "host" execution
label to reach the host's Docker daemon and filesystem.

docker-compose.yaml

@@ -0,0 +1,15 @@
+services:
+  maptool:
+    build: .
+    restart: unless-stopped
+    ports:
+      - "4567:4567"
+    environment:
+      BIND_HOST: 0.0.0.0
+      RACK_ENV: production
+    volumes:
+      # Uploaded GPX files and the offline PMTiles extract — kept outside
+      # the image so a rebuild/redeploy doesn't wipe them (see
+      # docs/development.adoc for how the deploy job treats this
+      # directory).
+      - ./data:/app/data