Document how to move from the Docker/Distribution registry to the Gitlab container registry

Context

This has been requested by at least one customer.

The GitLab Container Registry started as a fork of the Docker (now Distribution) container registry, available at https://github.com/distribution/distribution.

Since then, we've made a large number of changes, most notably those related to the new architecture based on the metadata database and the major enhancements it unlocked (docs). However, despite those changes, and ignoring the metadata database (which at the time of writing is disabled by default), our registry still remains compatible with the vast majority of the functionality supported by the upstream Distribution registry.

Therefore, either as a final destination or as an upgrade stop before enabling the metadata database, it's technically feasible for customers that are currently using the upstream registry to move on to the GitLab one with minimal changes required to their infrastructure and little to no downtime required.

Task

Add documentation to the GitLab container registry administration docs to help customers perform this transition.

We should review all breaking changes we've made to date and any missing features that may compromise this move. Off the top of my head, we have:

Technically speaking, provided that users find the settings they're currently relying on in our current configuration documentation, then there shouldn't be blockers to perform a drop-in replacement with no storage/data changes required. This should be performed with caution and validated on non-production environments first.

Edited by João Pereira