gitlab-ci: Use the v2021 stable docker images

The experimental backport of the gitlab pipeline was using images from the v2021dev2 development release.

Due to a GnuTLS bug triggered by a Let's Encrypt root CA certificate expiring some HTTPS requests to Apertis services are failing.

Switch to the v2021 stable docker images to benefit from the workaround that has been already landed there, see pkg/ca-certificates!14 (merged)