Docker build no longer works with github url

Change default URL for progrium/buildstep to include full git clone url
Merge pull request #405 from AlJohri/master

Fix docker build for docker v0.7.3
Add $DOCKER_VERSION to specify docker version
Suppress git advice related to detached HEAD
Merge pull request #400 from aegypius/suppress-git-output-unless-dokku-trace-is-enabled

Suppress git "detached HEAD" notice output
Merge pull request #407 from banyan/specify-docker-version

Add $DOCKER_VERSION to specify docker version
Merge branch 'master' into myupdated