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.
