Switch to python:3-slim docker base image

[jameshilliard]

I'm thinking it's probably a good idea to switch away from alpine images which are size optimized to python:3-slim based images which should have some decent performance advantages. AFAIU this is due to alpine images using musl libc which is size optimized instead the slim images which are glibc based and thus much more performance optimized. I don't think we have much of a reason to heavily optimize the docker images for size to the point where alpine images make sense.