Add an Alpine Linux variant

[andrewrabert]

Alpine-based images are typically much smaller than Debian-based ones.

[deniszh]

Not sure that alpine has support for all needed python modules, will check that. I have idea to decrease image size by using two stage build, but ubuntu image, though

[deniszh]

Ah, I see another problem here. Phusion baseimage using runit, and has quite specific environment, based on Ubuntu. Alpine has runit support now, but nothing else, so, porting to Alpine will require almost complete rewrite - see phusion/baseimage-docker#275

So, Apline support is possible, but will require some work, e.g. using https://github.com/just-containers/s6-overlay as base.

[deniszh]

PS: Please note that I switched this image to multistage now, dropping its size from 850 to 480MB - #56 (graphite itself uses 128MB, security updates - 178MB, rest is a base image)