Misc. Docker updates

Created by: muffato

Following Ensembl/ensembl-hive#137 git is not installed in the image any more. I have transformed the git clone into a separate build stage, which involves changing the build context to the root of the repo. An added benefit is that .git can be removed.

I have also commented out the step that installs the Perl dependencies of this repo since there are none at the moment.