Specifying only a branch like '2019-08' is problematic if upstream commits
something between the builds of two containers.
We could make a tarball in the first container build and reuse that, but we
need the flexibility of Git for some patching in the Android and JavaScript
builds, so instead we allow specifying a git tree-ish as the third argument.
We can therefore also give a pretty version string to use for the image name
as second argument, e.g. 6.6.0.160 for bef1e6335812d32f8eab648c0228fc624b9f8357.
As we can't `git clone` a commit hash directly, we still need to provide both
branch name and then commit hash for this use case.
The previous `['/bin/bash']` was not working, running the image
with `podman run -it <name>` would evaluate to:
```
> sh -c ['/bin/bash']
sh: [/bin/bash]: No such file or directory
```
This saves us from pulling gtk2 and gtk3 as weak deps of
perl and git or whatnot... And we don't need to pull in all
the documentation on Ubuntu.
Also ensures that we properly run `dnf clean all` after each
installation.
Cory Petkovsek
Rémi Verschelde
Hein-Pieter van Braam