Changes:
- uses debian:jessie as base image (more lightweight)
- .dockerignore .git/.tox to speed up build process considerably
- more caching-aware Dockerfile
- copy current directory instead of git cloning the repo inside the container
- /etc/letsencrypt and /var/lib/letsencrypt volumes;
no need for "if os.environ.get" hack
bootstrap script for debian had to be adjusted, as lsb_release is not
present in debian:jessie image.
- README has only the most important infos that a new reader needs in his first minute
of contact with the project (to decide whether it is interesting or not)
- CHANGES shall later be a curated change log (== important changes between releases)
- separate docs into intro, using, project
- intro docs = include README, CHANGES (avoid duplication)
