Fixes:
- patch applied to one seafdav file to make seafdav *start*
- export more environment variables to child processes so that they
  really are taken into account (like USE_EXISTING_DB, etc)
- use MEMCACHED_* environment variables to allow changing the server name
  from hardcoded value
- use 'ENABLE_WEBDAV' environment variable to enable/disable this feature
- on container start, ALWAYS update seahub_settings.py and seafdav.conf
  based on environment variables. This is a requirement to allow it
  to work inside a kubernetes cluster
- nginx configuration for webdav/seafdav changed to NOT use fastcgi mode
- small change in nginx configuration in case it is behind a SSL termination
  proxy

Others:
- a little bit of refactoring nginx related functions from bootstrap.py
- docker Makefile changed to take seafile version from the actual
  Dockerfile files, especially since standard and Pro versions can be
  different

OTHERS:
- updated README.md to contain information of the environment variables that can be used
- added a docker-compose/ directory with example to run
- updated README.md to refer to the current docker-compose/ example
- fix situation where an nginx reload could happen before nginx was started