server: Use multi-stage builds for ctags and syntect_server

Created by: keegancsmith

I have been iterating on our Dockerfile and was frustrated by invalidating ctags or the syntect_server lines. These are independent, so we can use docker's multi-stage builds to inform it as such.

Note: ctags should be its own Dockerfile. I'll save that change for a later PR where we open source more of our docker images from our infrastructure repository.