In case the systemwide default image changes, set the desired image
explicitly.

In fact, the default did recently change from `ubuntu:bionic` to
`suse-qt66:latest` from the local
`invent-registry.kde.org/sysadmin/ci-images` registry. This of course
contains no `apt-get` command, and `ubuntu:bionic` is out-of-support by
now. So start using `ubuntu:22.04` now.