Create smaller docker images using multi-stage build

[hardbyte]

Instead of including gcc and everything required to compile all our dependencies we could probably build a smaller image which would have a smaller attack surface.

https://docs.docker.com/develop/develop-images/multistage-build/#before-multi-stage-builds

[hardbyte]

Now that anonlink builds and deploys a universal wheel we probably don't need g++ or libffi-dev.

Unsure if need gmp-dev, mpfr-dev, mpc1-dev anymore too.

[hardbyte]

This looks like it could address the multi-stage build: https://tech.zarmory.com/2018/09/docker-multi-stage-builds-for-python-app.html

Another consideration - perhaps more important - is the container build time should be reduced.

[hardbyte]

Build time has been reduced in #506 would still be nice to have a smaller base image by using multi-stage. cc @wilko77