Fix issue #1 and lock Dockerfile to specific version of Sick Beard

The changes to start.sh fix #1 (I'm not familiar with the -b bash option for testing if a file is a block device, is that what you meant to use?  It seems to work better simply testing for the presence of the file)

The Dockerfile changes allow us to lock the docker image to a specific version of SickBeard (you could then tag the images when building and pushing to the docker repo.  Minor structural changes also reduce the number of intermediate images during build.