The current `make_container()` helper is a bit silly (which is
entirely my fault). It requires a container tag as input but all
tests end up creating a random number for this input. So instead
just remove the input and return the container_id from the podman
build in the contextmanager and use that.
This commit tweaks the excellent work from Gianluca in PR#1550 a
little bit. Now that the container inputs are their own input
type some of the code that used to be part of the original
`inputs/org.osbuild.containers` can be simplified.
