feat: set default registry

[No9]

Currently it seems that no default registry is set.
so FROM alpine fails to resolve.
There were conversations about this in podman and in the end they defaulted to docker.io to support Dockerfiles that were already in existence and also ensure tutorials Just Worked.
Not sure what the recommendation should be for FreeBSD but needs consideration.

[samuelkarp]

The container ecosystem at large generally assumes the default is docker.io.

[alice-sowerby]

@No9 are you happy to close this now?

[No9]

Hey @alice-sowerby
It not clear if this has been Implemented or not