Investigate CHT docker image size

[dianabarsan]

Describe the issue
Are our images too big, bloated with unnecessary dependencies?

Describe the improvement you'd like
Check images sizes, and if we mistakenly package unnecessary bits and remove them.

[garethbowen]

I read a lot on the interwebs about docker images being unnecessarily large and there are a bunch of blog posts about how to optimise them, eg: https://medium.com/@reach2shristi.81/real-life-proven-strategies-to-reduce-docker-image-size-0415b756f886

We have done very little optimisation work here so I expect there's a load of low hanging fruit...