import major modules when building docker instance

[chvp]

From https://github.ugent.be/dodona/dodona/issues/389

Importing larger modules like numpy, scipy, pandas takes some time if they haven't been imported before (Python stores compiled versions on disk). Since these modules have not been imported before in the docker instance, each time it gets started, the import of the modules gives some overhead. Could be bybpassed by importing some of these modules when building the docker.

Original issue by @charvp on Thu Sep 27 2018 at 14:36.