Print an immutable reference to repository config sha256

What does this MR do?

Adds immutable reference to docker registry image used for gitlab-ci.

Why was this MR needed?

If not available, there is no way to reproduce a reproducably fetch the exact version of an image used by gitlab-ci.

Are there points in the code the reviewer needs to double check?

This is my first attempt at a go contribution, so probably yes. All of it.

What are the relevant issue numbers?

This is a fix for issue #4268 (closed)