[FLOC-1012] The docs.clusterhq.com docker image recipe

Imported rtd-https-proxy from https://github.com/msh100/Docker-Images

Using git instructions from http://gbayer.com/development/moving-files-from-one-git-repository-to-another-preserving-history/ to preserve @msh100 's commit history.

I've built and tagged this image as `clusterhq/docs.clusterhq.com` and published to https://registry.hub.docker.com/u/clusterhq/docs.clusterhq.com/

To test:
```
docker run -v $PWD/cert/:/ssl/ -80:80 -p 8443:443 -e "DOCS=flocker" clusterhq/docs.clusterhq.com
```

```
curl --verbose --resolve flocker.readthedocs.org:80:127.0.0.1 http://flocker.readthedocs.org/ > /dev/null
```

Fixes https://clusterhq.atlassian.net/browse/FLOC-1012