[DOP-15023] Fix backend entrypoint

Author: dolfinus

docker/entrypoint_backend.sh

@@ -4,4 +4,4 @@ set -e
 python -m syncmaster.db.migrations upgrade head
 
 # exec is required to forward all signals to the main process
-exec python -m syncmaster.backend --host 0.0.0.0 --port 8000 "$@" &
+exec python -m syncmaster.backend --host 0.0.0.0 --port 8000 "$@"