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.

.dockerignore

@@ -0,0 +1,9 @@
+# this file uses slightly different syntax that .gitignore,
+# e.g. ".tox/" will not ignore .tox directory
+
+# well, official docker build should be done on clean git checkout
+# anyway, so .tox should be empty... But I'm sure people will try to
+# test docker on their git working directories.
+
+.git
+.tox