Make `docker` executable-name configurable.

[mstone]

Situation

Debian has decided to name their docker executable docker.io instead of docker and, as a result, the docker-run scripts produced by nix-docker do not run properly on Debian + nix-1.6.1.

Workarounds

So far, patching the runContainerScript attribute of nix-docker/docker.nix has been sufficient to let me further explore nix-docker.

Questions

1. Would it be appropriate to make the (name of | path to) the docker executable configurable at nix-docker build time?
2. If so, should default.nix grow a patchPhase to incorporate this information into the runContainerScript attribute value?
3. Alternately, is there a better way to accommodate Debian's variation in naming?