Make builds reproducible

[powerjg]

We want to make sure everyone generates the exact same binary when they compile. For now, Docker images would probably be the easiest way to do this. Make sure it include the Dockerfiles in this repo so others can make the docker images.

BTW, make sure to specify the exact tag in the FROM statement. Otherwise, the base of the docker image could be changed without you knowing if it's on Dockerhub (well, actually, even with the tag it could be changed, but it's less likely).

[hnpl]

Docker has official image for gcc for X86 and ARM, but I couldn't find the official one for RISCV. However, I found this https://hub.docker.com/r/ucbbar/riscv-docker-images. I don't really know whether they are the UCB-BAR. Should we aim to provide a Docker file for all targets?

[powerjg]

A couple of things.

1. What does "docker official" image mean? Does this tag/image ever change? If so, we should make sure to save the exact image we're using somewhere.
2. UCB-BAR is the Berkeley group. That image should work.

However, we might just want to quickly roll our own. It would probably be good experience for you, and it would guarantee we knew what was in the images.

[hnpl]

See #5