Merge "container role docs : clarify requirements"

roles/build-container-image/common.rst

@@ -112,9 +112,10 @@ remote registry.
 
 *Dependencies*
 
-Use the :zuul:role:`ensure-skopeo` role as well as the
-:zuul:role:`ensure-docker`, or :zuul:role:`ensure-podman` roles before
-using the roles described here.
+The build and upload roles require a container runtime that should be
+installed before use; for example by using either the
+:zuul:role:`ensure-docker` or :zuul:role:`ensure-podman` roles.  The
+promote job assumes `skopeo` is available on the executor.
 
 **Role Variables**