Add Docker build & push to CI

Adds a step to the CI script from the no-integration branch that builds and then pushes a docker image to our hosted gitlab container registry. It will push the image with the latest tag on master and [branchname]-latest tag on all other branches.

Successful run example: https://gitlab.futo.org/videostreaming/verifiers/-/pipelines/1333

Please merge MR !2 (closed) before this, or merge this in place of that one.