Adjust .gitlab-ci.yml to use correct registry when forking

Context

CI Build

Summary

The predefined variables section contains a hardcoded repository location which is not possible to push to in forked projects.

Steps to reproduce

Fork this project, make a commit and see the preparation step failing :(

What is the current issue behavior?

Pushing to the base projects registry fails, See !22 (merged)

What is the expected correct behavior?

A push to the forked projects repository should succeed.

Relevant logs and/or screenshots

--

Possible fixes

We should probably use CI_REGISTRY_IMAGE instead of the hardcoded string.

See available CI Variables