Add Docker build and Gitea Actions CI for deployment

Multi-stage Alpine Dockerfile producing a standalone Next.js image
with the better-sqlite3 native addon included. Gitea workflow builds
on push to main and on v* tags, pushing to the Gitea registry using
the REGISTRY_URL variable and REGISTRY_TOKEN secret. Compose file
mounts ./data so alerts.db and settings.json persist across restarts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,24 @@
+services:
+  oversnitch:
+    image: ${REGISTRY_URL:?set REGISTRY_URL to your Gitea host, e.g. gitea.example.com}/${IMAGE_REPO:-josh/oversnitch}:${IMAGE_TAG:-latest}
+    container_name: oversnitch
+    restart: unless-stopped
+    ports:
+      - "${HOST_PORT:-3000}:3000"
+    volumes:
+      # Persists alerts.db and settings.json across restarts
+      - ./data:/app/data
+    environment:
+      - NODE_ENV=production
+      # All service URLs/API keys can be configured via the Settings UI after
+      # first boot. Uncomment below to provide them via env instead.
+      # - SEERR_URL=
+      # - SEERR_API=
+      # - RADARR_URL=
+      # - RADARR_API=
+      # - SONARR_URL=
+      # - SONARR_API=
+      # - TAUTULLI_URL=
+      # - TAUTULLI_API=
+      # - DISCORD_WEBHOOK=
+      # - NODE_TLS_REJECT_UNAUTHORIZED=0