`matrix_container_global_registry_prefix_override` may look similar to
the old `matrix_container_global_registry_prefix` variable
(removed in d6bf789710), but it's different.

The old `matrix_container_global_registry_prefix` variable was just a hardcode of `docker.io/` and roles that
needed to refer to `docker.io/` could use it. However, this was:

- not used by all roles, because some need another registry (not `docker.io/`)
- used only by roles within the playbook (`roles/custom`), not external roles

Overriding the old `matrix_container_global_registry_prefix` variable was rather pointless,
as it didn't cover everything.

The new `matrix_container_global_registry_prefix_override` variable, on the other hand,
lets you override the registry prefix for all components, regardless of whether they
use `docker.io/` or another registry by default.

This is useful to people who have somehow mirrored all container images to their own registry,
as it provides them with a single variable they can flip to influence the whole playbook.