registry db-migrations container isn't rendering config correctly

Summary

When enabling the registry database in the pre env (registry.database.enabled: true) with this MR, the configuration for the registry container is rendered correctly and working, but the db-migrations job container is failing with:

configuration error: parsing /etc/docker/registry/config.yml: yaml: unmarshal errors:
line 53: cannot unmarshal !!str `SECRET_...` into []string
Usage:
registry database migrate status [flags]

I suspect it's leaving the dummy SECRET_... string in there because of a missing secret here but I would have thought that this part of the template isn't rendered at all because of this.

This is using the gitlab chart version b8973d3b27a963624b3cc7ba294f7b1c95278f9f

/cc @mnielsen @jdrpereira