docker daemon http proxy and quickstart

[msegura501]

Many organizations are using an http proxy so that direct access is not possible.

In this configuration, many docker default setup are making the proxy directly available within containers (see here).

This creates a situation where caddy tries to go the configured http proxy provided by the docker daemon instead of using a direct connection to the other containers.

[todpunk]

I personally think that's something the environment should be aware of and overriding for, not Artifact Keeper, mostly because in this case it might be nice to cater to the edge case, but the cost is that now behavior is weird when you expect it to honor the common proxy env vars. I'll put a possibly change to help both cases without having them fight in the PR the references this.