Feature versioning

The partial merge of my previous changes left your repo in a bad state (curl was downloading a file called "master" and then tar was looking for a file called "sickbeard.tgz".  Here's a working of the Dockerfile that pulls from master, as it originally did - but contains a variable (set to 'master') and an example line to help guide users who might want to pick a specific build/tag.  

I've tried to better isolate my changes this time, you should be able to merge either/both PR as desired.

I understand the rationale behind not wanting to keep up with Sick Beard releases - it's a stable enough project that I am also lax about keeping up with new releases.