NG: registry debug tools

[dmp42]

We probably want to ship a set of helpers that would make reporting bugs and/or diagnosing easier.

On top of the head:

• something to run network sanity tests and output a compact summary (dns resolution, ping latency, speed test measurement, sthing else?)
• something to introspect and extract meaningful informations from the process / other helpers to maybe restart a registry in debug mode?
• backend driver sanity report
• dependency versions, extensions/backend driver version, system versions, os, kernel (?)
• running configuration dump (possibly magically redacting secrets?)
• standalone client tools to perform API tests requests to the registry
• tools to extract registry related informations from the docker engine?
• else?

I'm flagging as extension for now, though that doesn't have to be a "true" extension in itself (like "true APIs" :)).

cc @BrianBland @samalba

[stevvooe]

Superseded by docker/distribution#/198.