During the work on PR#1752 Florian discovered that make_containers()
is broken for nested containers like:
```
with make_container(tmp_path, {"file1": "file1 from base"}) as base_tag:
with make_container(tmp_path, {"file1": "file1 from final layer"}, base_tag) as cont_tag:
```
It errors with:
```
Error: 5b947de461ee21b858dd5b4224e80442b2f65b6410189147f2445884d9e4e3d8: image not known
```
The reason is that we work with hashes for the image and then call
`podman image rm` which by default will also remove all dangling
references. Those are defined by not having a tag and not referenced
anymore. So the inner container cleanup also removes the outter.
There are many ways to fix this, I went with re-adding tags to the
test containers because it also makes it easy for the user to see if
we left any containers (accidently) around.
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.
