[Docker] Fix ssh zombie processes issue

SSHTunnel.open() starts the ssh process with -f option, which
causes it to become a daemon (it calls daemon(3) after authentication).

Then, SSHTunnel.close() uses `ssh -O exit` to ask that daemon process
to exit (the processes communicate via the control socket, -S option).

The daemon exits, and then PID 1 should reap it.

Before #3165, PID 1 was bash,
which handles SIGCHLD, properly wait(2)ing children, but in that
pull request exec was added, making `dstack server` PID 1. dstack
does nothing with SIGCHLD (neither handles nor explicitly ignores
it), thus the default disposition (ignore the signal but do not discard
children) leads to an evergrowing number of unreaped zombies.

Fixes: #3291

Author: un-def

docker/server/entrypoint.sh

@@ -10,7 +10,7 @@ fi
 DB_PATH="${HOME}/.dstack/server/data/sqlite.db"
 mkdir -p "$(dirname "$DB_PATH")"
 if [[ -z "${LITESTREAM_REPLICA_URL}" ]]; then
-  exec dstack server --host 0.0.0.0
+  dstack server --host 0.0.0.0
 else
   if [[ ! -f "$DB_PATH" ]]; then
     echo "Attempting Litestream restore..."
@@ -23,5 +23,5 @@ else
       fi
     fi
   fi
-  exec litestream replicate -exec "dstack server --host 0.0.0.0" "$DB_PATH" "$LITESTREAM_REPLICA_URL"
+  litestream replicate -exec "dstack server --host 0.0.0.0" "$DB_PATH" "$LITESTREAM_REPLICA_URL"
 fi