Persist SSH host key across restarts

- Remove key generation from Dockerfile (was causing new key each build)
- Add ssh_keys volume mount in docker-compose
- Generate key at runtime only if it doesn't exist
- No more clearing known_hosts after updates!

docker-compose.yml

@@ -8,6 +8,8 @@ services:
       - SHELLMATE_REDIS_URL=redis://redis:6379
       - SHELLMATE_DATABASE_URL=postgresql://shellmate:shellmate@postgres:5432/shellmate
       - STOCKFISH_PATH=/usr/games/stockfish
+    volumes:
+      - ssh_keys:/etc/shellmate
     depends_on:
       - redis
       - postgres
@@ -47,3 +49,4 @@ services:
 volumes:
   redis_data:
   postgres_data:
+  ssh_keys: