docker: default to monolithic mode

Otherwise, if you try to use the api-server before db migrations have
happened, you'll get the very helpful (sarcasm) error when running `attic`:

    Error: InternalServerError: The server encountered an internal error or misconfiguration.

With this corresponding error from `atticd`:

    Database error: Query Error: error returned from database: relation "cache" does not exist

flake.nix

@@ -100,7 +100,6 @@
         ];
         config = {
           Entrypoint = [ "${packages.attic-server}/bin/atticd" ];
-          Cmd = [ "--mode" "api-server" ];
           Env = [
             "SSL_CERT_FILE=${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt"
           ];