refactor Dockerfile

[just-simon-5]

Dear Ensembl,

Recently while working on an internal project at my work, I had to refactor our VEP Dockerfile which is based on your provided Dockerfile.

So I thought I would share those changes with you and you can see if you want to use them or not!

Essentially I tried to extract all the different commands from various scripts used by this Dockerfile into the Dockerfile itself, I also attempted to pin down versions of every package and refactor the large, complex RUN block used in the builder image.

Please note:
I did not set up the USER or permissions for that USER.
I specify the fully list of plugins to install rather than using all as the gwava plugin has an problems (link to issue in comment).

Feel free to disregard!

Thanks,
Simon