checkmake CI job fails for forks

Job #1474981155 failed for bc971ed1:

ERROR: Job failed: failed to pull image "registry.gitlab.com/dschalla/gitlab-development-kit/asdf-bootstrapped-verify:main" with specified policies [always]: Error response from daemon: manifest for registry.gitlab.com/dschalla/gitlab-development-kit/asdf-bootstrapped-verify:main not found: manifest unknown: manifest unknown (manager.go:205:0s)

We need to update the checkmake CI job definition to use a publicly accessible Docker image.