Improve reproducibility of builds #42
Description
The current Dockerfile.test file has a lot of time-dependent commands in it which make the current build non-reproducible. If multiple team members were working together on the same 'bot, they would no doubt be tripped-up by this problem.
This problem can be fixed by publishing to ECR (or another Docker registry) these time-dependent layers and having multiple team-members reference them by a name and version, which is a reproducible way of sharing these lower layers.