Smaller docker image

Created by: muffato

Like in Ensembl/ensembl-hive#137 I have transformed the Dockerfile to do a multi-stages build. Overall it brings a clean separation between several stages of the builds, and allows only selecting the relevant files from each stage without having to clean stuff up in the same command with &&. The resulting image is now just 70MB heavier than the parent eHive image instead of 166MB before, and better utilizes the build cache. Note that the build context is also expected to be different and I will have to change this on the Docker Hub