ci: Update Docker image repository

Docker images are no longer published to a privately managed DockerHub
account to avoid sharing a password. DockerHub also has a restriction on
the number of Docker images that can be stored for private accounts.

As the Docker image repository has been moved from DockerHub to GitLab
registry, we must fetch our Docker images from the new repository
location. If we don't, CI will break as the Docker images we depend on
will not be available.

Signed-off-by: Saheer Babu <saheer.babu@arm.com>