Add support for pulling images from private registries

[mrBliss]

Fixes #82.

pullImage now takes a PullOpts argument, which includes an optional AuthConfig. For more details, see: https://docs.docker.com/engine/api/v1.41/#section/Authentication

[mrBliss]

It seems this project has no CI. Before submitting this PR, I successfully compiled this branch with all stack-*.yaml files in this repo (hence the choice for base64-bytestring library over the newer base64 library, which does not support GHC 7.10, IIRC).

I didn't add a test, as it would require setting up a private Docker registry, which is quite the effort. However, I did successfully test it with a private registry (using Credentials).