Support /etc/docker/certs.d/

Docker enables folks to use custom certs for their registries via this directory (and possibly environment variables). We should figure out how tools can support this, perhaps through support directly in the library, or perhaps through documentation on how to properly set up the `http.RoundTripper` passed into `remote.Foo` calls.

`crane` and `ko` would be good examples for how we add support for this to tools.