Put ASSETS_IMAGE in to fix Unicorn error

I set the ASSETS_IMAGE arg to the gitlab registry for now. I expect the team will need to put it in the base registry.

The contents of the image is derived the tar.gz attached to this job: https://gitlab.com/gitlab-org/gitlab/-/jobs/341125819

Each merge into the master branch has a job that compiles and tars the assets. A follow-on job wraps it in a dockerfile.

This will be addressed for DSOP in the offline build requirements that are being worked on now.