gitlab: centralize the container tag name

gitlab: centralize the container tag name

We use a fixed container tag of 'latest' so that contributors' forks
don't end up with an ever growing number of containers as they work
on throwaway feature branches.

This fixed tag causes problems running CI upstream in stable staging
branches, however, because the stable staging branch will publish old
container content that clashes with that needed by primary staging
branch. This makes it impossible to reliably run CI pipelines in
parallel in upstream for different staging branches.

This introduces $QEMU_CI_CONTAINER_TAG global variable as a way to
change which tag container publishing uses. Initially it can be set
by contributors as a git push option if they want to override the
default use of 'latest' eg

  git push gitlab <branch> -o ci.variable=QEMU_CONTAINER_TAG=fish

this is useful if contributors need to run pipelines for different
branches concurrently in their forks.

Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230608164018.2520330-2-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>