Conda-docker

**Changed:**

* `build_docker_environment` refactored into two functions to expose
an internal function `build_docker_environment_image` to be used by
libraries e.g. conda-store

**Fixed:**

* Use copies if conda installation and temporary directory are on different devices
* JSONDecodeError when using a list of package specification

**Authors:**

* Anthony Scopatz
* Chris Ostrouchov
* Chris Burr
* Julian Rüth

**Added:**

* Now implements a one-layer-per-package option. There is the 125 layer limit
in docker, so for safety we only allow 100 individiual package layers. All
packages after the initial 100 are combined into a single, last, squashed layer.
The packages are installed in dependency order, so base-level packages are
more likely to get their own layer and be reused. This is inspired by
https://grahamc.com/blog/nix-and-layered-docker-images

**Authors:**

* Anthony Scopatz
* Chris Ostrouchov


**Added:**

* Adding support for setting docker v1 layer config e.g. ENV, LABEL, CMD, ENTRYPOINT etc.

**Added:**

* Initial release of `conda-docker`!

**Authors:**

* Anthony Scopatz