Use multi-stage build to create CI container

[jbush001]

Currently, the build container is created by copying files from the host install, which is messy and not fully reproducible:

https://github.com/jbush001/NyuziProcessor/blob/master/scripts/build_container.sh

Instead, use the new multi-stage build feature to Docker to build the compiler and Verilator inside a container:

https://docs.docker.com/develop/develop-images/multistage-build/

[jbush001]

Also, should use build output artifacts from jbush001/NyuziToolchain#107 instead of having to rebuild the compiler.