Add registry authentication tests

[nu12]

As of now we have no tests for registry authentication. Methods that need to be tested are:

• Image#push
• Image#build
• Image#create
• Image#distribution

Possible approaches: a) setup a local registry with authentication, b) stub the request/response.

[zarqman]

As asked in #3, and this might already be what you meant in (a) above, but what about creating an instance of docker/registry inside the being-tested docker daemon? You're already pushing images and creating containers there, so this is just another image + container in that regard. (This is different that requiring a pre-configured registry instance running external to the test suite.)

[nu12]

That's what I had in mind, but I thought I'd ask a second opinion. For testing the authentication itself this registry instance needs extra configuration such as htpasswd and TLS (reference). I'm thinking about creating those files using bin/setup and then use them during tests.