Add gid to config.toml only when docker group is present

If we don't install docker and install just containerd apt packages,
there is no docker group. In this scenario, we should not add the gid to
config.toml

Author: dims

cluster/gce/gci/configure-helper.sh

@@ -2668,8 +2668,6 @@ EOF
       cni_template_path=""
     fi
   fi
-  # Reuse docker group for containerd.
-  local containerd_gid="$(cat /etc/group | grep ^docker: | cut -d: -f 3)"
   cat > "${config_path}" <<EOF
 # Kubernetes doesn't use containerd restart manager.
 disabled_plugins = ["restart"]
@@ -2678,9 +2676,6 @@ oom_score = -999
 [debug]
   level = "${CONTAINERD_LOG_LEVEL:-"info"}"
 
-[grpc]
-  gid = ${containerd_gid}
-
 [plugins.cri]
   stream_server_address = "127.0.0.1"
   max_container_log_line_size = ${CONTAINERD_MAX_CONTAINER_LOG_LINE:-262144}
@@ -2691,6 +2686,16 @@ oom_score = -999
 [plugins.cri.registry.mirrors."docker.io"]
   endpoint = ["https://mirror.gcr.io","https://registry-1.docker.io"]
 EOF
+
+  # Reuse docker group for containerd.
+  local containerd_gid="$(cat /etc/group | grep ^docker: | cut -d: -f 3)"
+  if [[ ! -z "${containerd_gid:-}" ]]; then
+    cat >> "${config_path}" <<EOF
+# reuse id of the docker group
+[grpc]
+  gid = ${containerd_gid}
+EOF
+  fi
   chmod 644 "${config_path}"
 
   echo "Restart containerd to load the config change"