Set up CI caching

[andersfischernielsen]

This will:

• modify CI runs to use npm instead of yarn (caching in yarn is a nightmare to set up, I tried)
• add creation of intermediate docker images for caching (these should be cached by the action-docker-layer-caching action)
• modify orders of Dockerfile statements to improve caching slightly

[andersfischernielsen]

It looks like this has sped up the CI runs and local docker builds. Merging!