Docker images are way too big

[telliere]

Container preparation image is 1.2GB, data preparation is 1.3GB.

Leads :

Squash images (less layers might improve this issue)
Use a lighter base image (currently using python-debian)

(This issue is imported from the original private repository)

[telliere]

Change of base image (python-debian -> python-alpine) didn't change a lot even though implemented in : 61629ab