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.
When an images does not exist just return `False` instead of
raising a RuntimeError. If anything else goes wrong (unknown
output or hash mismatch) keep the RuntimeError as this is an
unexpected exception.
