Try flux against an image registry that uses basic auth

[squaremo]

It's possible to deploy your own image registry and secure it with basic auth. I think that flux assumes you are using token-based auth, and will fail to fetch image metadata from a registry using basic auth. So we should try this out by running a local registry and turning --registry-trace on to see what happens.

[squaremo]

The scenario prompting this issue is a docker-registry with a reverse proxy (nginx) in front of it; the proxy does the auth, for every path under /v2.