docs: add user-facing README and update compose for production

Add README with Docker setup instructions for self-hosting. Update
docker-compose.yml to use the pre-built registry image instead of
local build, and add a healthcheck against /api/health.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   gearbox:
-    build: .
+    image: gitea.jeanlucmakiola.de/makiolaj/gearbox:latest
     container_name: gearbox
     ports:
       - "3000:3000"
@@ -10,6 +10,12 @@ services:
     volumes:
       - gearbox-data:/app/data
       - gearbox-uploads:/app/uploads
+    healthcheck:
+      test: ["CMD", "bun", "-e", "fetch('http://localhost:3000/api/health').then(r=>r.ok?process.exit(0):process.exit(1)).catch(()=>process.exit(1))"]
+      interval: 30s
+      timeout: 5s
+      start_period: 10s
+      retries: 3
     restart: unless-stopped
 
 volumes: