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.

docs/using.rst

@@ -49,13 +49,20 @@ Mac OSX
 
 Quick Usage
 ===========
-Using docker you can quickly get yourself a testing cert.  From the server that the domain your requesting a cert for resolves to, download docker 1.5, and issue the following command:
 
-::
+Using docker you can quickly get yourself a testing cert. From the
+server that the domain your requesting a cert for resolves to,
+download docker, and issue the following command
 
-    docker run -it --rm -p 443:443 -v $PWD/certs/:/etc/letsencrypt/certs/ letsencrypt/lets-encrypt-preview
+.. code-block:: shell
 
-And follow the instructions.  Your new cert will be available in `certs/`
+   sudo docker run -it --rm -p 443:443 \
+               -v "/etc/letsenecrypt:/etc/letsencrypt" \
+               -v "/var/lib/letsenecrypt:/var/lib/letsencrypt" \
+               letsencrypt/lets-encrypt-preview
+
+And follow the instructions. Your new cert will be available in
+``/etc/letsencrypt/certs``.
 
 Installation
 ============