containerignore: allow content-set to be copied

This can be removed when we have https://gitlab.com/fedora/bootc/tekton-catalog/-/merge_requests/13

Author: jbtrystram

.containerignore

@@ -17,3 +17,4 @@
 !/overlay.d/
 !/overrides/
 !/platforms.yaml
+!/content-sets.json