Build in docker image

[sbellem]

We need a docker image with Coq 8.3p15. The docker images for Coq at https://www.github.com/coq-community/docker-coq only support 8.4 and up.

[sbellem]

Perhaps will give this up as there's a repo at https://github.com/EasyCrypt/certicrypt and a branch for 8.5 at https://github.com/EasyCrypt/certicrypt/tree/8.5.