feat: allow prefix to image names

This is in order to support proxy/caching without
needing to maintain separate inventory override of
each and every image.

Closes-Bug: CLOUDOPS-761
Change-Id: I7585377939ac7e6757f1cdbcfcdffae02bab2715
diff --git a/tox.ini b/tox.ini
index 44a886c..0673da3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -21,7 +21,9 @@
   {posargs}
 
 [testenv:pin-digests]
+skip_install = true
 deps =
+  docker-image-py>=0.1.12
   oslo_config
   oslo_log
   ruyaml