Added support for group_add key

This PR will add support for `group_add` key which will map to
supplemental group in pod security context.

pkg/loader/compose/v1v2.go

@@ -264,6 +264,14 @@ func libComposeToKomposeMapping(composeObject *project.Project) (kobject.Kompose
 		serviceConfig.MemLimit = composeServiceConfig.MemLimit
 		serviceConfig.TmpFs = composeServiceConfig.Tmpfs
 		serviceConfig.StopGracePeriod = composeServiceConfig.StopGracePeriod
+
+		// Get GroupAdd, group should be mentioned in gid format but not the group name
+		groupAdd, err := getGroupAdd(composeServiceConfig.GroupAdd)
+		if err != nil {
+			return kobject.KomposeObject{}, errors.Wrap(err, "GroupAdd should be mentioned in gid format, not a group name")
+		}
+		serviceConfig.GroupAdd = groupAdd
+
 		komposeObject.ServiceConfigs[normalizeServiceNames(name)] = serviceConfig
 		if normalizeServiceNames(name) != name {
 			log.Infof("Service name in docker-compose has been changed from %q to %q", name, normalizeServiceNames(name))
@@ -390,3 +398,17 @@ func getVol(toFind kobject.Volumes, Vols []kobject.Volumes) (bool, kobject.Volum
 	}
 	return false, kobject.Volumes{}
 }
+
+// getGroupAdd will return group in int64 format
+func getGroupAdd(group []string) ([]int64, error) {
+	var groupAdd []int64
+	for _, i := range group {
+		j, err := strconv.Atoi(i)
+		if err != nil {
+			return nil, errors.Wrap(err, "unable to get group_add key")
+		}
+		groupAdd = append(groupAdd, int64(j))
+
+	}
+	return groupAdd, nil
+}