We build the containers by running an install script. When the
script changes, we usually want to re-generate the containers.

That is easy to miss. We could improve that by embedding a checksum
of the file as a comment in ".gitlab-ci.yml"

   {% for distro in distributions %}
     {{"%-13s"| format(distro.name.upper() + '_EXEC:')}}'bash .gitlab-ci/{{distro.name}}-install.sh' # {{ ci_fairy.hashfiles('.gitlab-ci/' + distro.name + '-install.sh') }}
   {% endfor %}

which results in

   FEDORA_EXEC: 'bash .gitlab-ci/fedora-install.sh' # db512ed0d3f6880a01ab29cba42abc233f56a7eab1f8ee9a83031f9244b84288
   UBUNTU_EXEC: 'bash .gitlab-ci/ubuntu-install.sh' # 6e35a936aba079f4d91eece147f77d3f65cdc562308a680018a2035a6efdbf2e

As we have a test that compares ".gitlab-ci.yml" to the output of
`ci-fairy generate-template`, the comment would be a reminder that
we should bump the tag and re-generate ".gitlab-ci.yml".

Alternatively, we could make the hash part of the distro-tag

   {% for distro in distributions %}
     {{"%-13s"| format(distro.name.upper() + '_TAG:')}}'{{distro.tag}}-{{
       (ci_fairy.hashfiles('.gitlab-ci/config.yml',
                           '.gitlab-ci/ci.template',
                           '.gitlab-ci/' + distro.name + '-install.sh'))[0:12]
     }}'
   {% endfor %}

which results in

   FEDORA_TAG:  '2020-11-10.0-b137473ad957'
   UBUNTU_TAG:  '2020-11-10.0-542ace328cfb'

Signed-off-by: Thomas Haller <thaller@redhat.com>