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!

Dockerfile

@@ -20,12 +20,10 @@ RUN useradd -m shellmate && \
     mkdir -p /etc/shellmate && \
     chown shellmate:shellmate /etc/shellmate
 
-# Switch to shellmate user and generate SSH key
-USER shellmate
-RUN ssh-keygen -t ed25519 -f /etc/shellmate/ssh_host_key -N ""
-
 EXPOSE 22
 
+# Key is generated at runtime (see entrypoint) or mounted from volume
+
 ENV STOCKFISH_PATH=/usr/games/stockfish
 
 CMD ["shellmate-server"]