service: Support setting container user

roles/service/meta/argument_specs.yaml

@@ -25,6 +25,11 @@ argument_specs:
         description: "The image to run in the service container(s), in FQIN format (registry/imagename:tag)."
         type: str
         required: true
+      service_container_user:
+        description: The UID to run as inside the container
+        type: str
+        required: false
+        default: ""
       service_container_publish_ports:
         description: "A list of published ports in docker format (<host listen address>:<host port>:<container port>)"
         type: list