Slim down CI image

Description

The current CI image currently sits at 1.25GB. We can remove ~120MB from the final image size. We're also sending 500MB of Docker context to the build, when all the build needs are the Makefiles (being fixed by !2138 (closed)).

Proposal

• Remove extraneous package content by passing --no-install-recommends
• Make use of multi-stage builds
• Reduce number of layers

Links to related issues and merge requests / references