This change adds granular, non-docker daemon docker image fetchers and
a docker image layer compositor to be used in conjunction with the
`docker2nix` utility provided by the `haskellPackages.hocker` package.

This change includes a hackage package version bump and updated sha256
for recent fixes released to `hocker` resulting from formulating this
patch.

This change adds a simple integration test exercising the fetchdocker
Nix code and hocker utilities for the simple `hello-world` docker
container. We exercise:

- Fetching the docker image configuration json
- Fetching the docker image layers
- Building a compositor script
- Loading the `hello-world` docker image into docker using the
  compositor script and `docker load`
- Running that loaded container

…s into parnell/fetchdocker

Support fetching docker images from V2 registries