buildah: BUG: tagging is broken for multistage builds

[Now that I have access to the nightly ppa’s (cf. containers/libpod#2250), I’m starting to file some issues against podman - and since the issue template there says that podman build issues should be filed for buildah, I’m starting here.]

Assume I have a toy dockerfile, say podman_test.dockrf:

FROM docker.io/library/ubuntu:bionic

RUN apt-get update && apt-get install curl -y

CMD ["bash"]

Then sudo podman build -t podman_test -f podman_test.dockrf . produces

So far so good. If I change the dockerfile slightly:

-FROM docker.io/library/ubuntu:bionic
+FROM docker.io/library/ubuntu:bionic as base

 RUN apt-get update && apt-get install curl -y

 CMD ["bash"]

then I get an untagged image (even though it successfully uses the cache):

Same for sudo podman build -t podman_test --target=base -f podman_test.dockrf .

This is quite basic functionality IMO - without tagging, many automation tasks are not possible.