This will hoist even more code into util out of the skopeo stage.
Now a caller can call:
with containers.container_source(image) as (image_name, image_source):
print(f"{image_name}, {image_source}")
to process containers inputs.
This hoists container handling code from the skopeo stage into
util/containers. It is prep for adding another stage that accepts
containers as an input. The code is common so we should share it
amongst all stages that use containers as input.