Document HTTP/HTTPS pitfall when pulling from the local Docker registry

Summary:
Pulling from localhost:5000 fails with "server gave HTTP response to
HTTPS client" because that endpoint is plain HTTP while Docker expects
HTTPS by default. registry.nasqueron.org is the same registry behind
our HTTPS reverse proxy and works everywhere without extra config, so
recommend it for pulls, including from docker-001.

Refs T1792

Reviewers: dereckson, ptdradmin

Reviewed By: dereckson, ptdradmin

Maniphest Tasks: T1792

Differential Revision: https://devcentral.nasqueron.org/D4131