Document registry support for old docker versions
The compatibility_schema1_enabled
configuration flag was added in !3061 (merged) in order to allow admins to turn on the compatibility api for their instance. (off by default in the latest version) This settings enables support for older docker clients to connect and push images: #4145 (closed)
We need to add this setting to the our container registry admin documentation: https://docs.gitlab.com/ce/administration/container_registry.html