Use the full image tag (dangerzone.rocks/dangerzone:latest) when
building the image. Else, we risk creating a `share/image-id.txt` file
with multiple IDs in it, if we have another
`dangerzone.rocks/dangerzone` image (with a different tag) in our dev
environment.
Skip the creation of the `share/container.tar` file, since it's not used
anywhere. Instead, pipe our `docker/podman save` invocations to `gzip`
directly, which will compress the tarfile on the fly. This saves both
time and disk space.
