Gitlab CI: use the docker:stable image for image creation

alpine:latest which is now the default image doesn't docker installed by
default. apk add docker results in failures
	Warning: failed to get default registry endpoint from daemon

last time all this worked was when we defaulted to the docker:stable image,
see https://gitlab.freedesktop.org/libinput/libinput/-/jobs/8316



Let's switch back to that and move on with our lives.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>