Update docker setup.

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.

docker-compose.yml

@@ -3,4 +3,5 @@ letsencrypt:
   ports:
     - "443:443"
   volumes:
-    - ./certs/:/etc/letsencrypt/certs/
+    - /etc/letsencrypt:/etc/letsencrypt/certs
+    - /var/lib/letsenecrypt:/var/lib/letsenecrypt