Fix centos toolbox container base image

[phlogistonjohn]

In PR #135 I added a comment to images/toolbox/Containerfile.centos:

# FIXME - this is not a real tag publicly available in the
# quay.io/samba.org/samba-client repository. This only works if you build
# the centos client locally first or acquire the image from a side channel.
# This needs to be converted to something public and/or configurable
# later.
FROM quay.io/samba.org/samba-client:centos-latest

Because the "centos-latest" tag is not universally available. It only ever exists locally on a system that first builds the centos client image. Thus the quay.io/samba.org/ part is a bit of a fib. This issue exists to discuss and do something about this

---

          > This raise two questions:

1. What is the appropriate tag we should use?

I don't know all I know is this will break in some circumstances. But I didn't want this PR to become sidetracked by that issue so I didn't try to fix it, just left a breadcrumb to follow up on later.

2. Should we still use `samba-client` as base image for `toolbox` or maybe use other (centos,fedora,opensuse) as base?

I think layering the images is fine, but there are some downsides to it. Let's discuss this more in a new issue or meeting, etc. :-)

Originally posted by @phlogistonjohn in #135 (comment)

[obnoxxx]

@phlogistonjohn might a multi-stage build containerfile help with this?