Add a riscv toolchain to Dockerfile

[fgaz]

The ubuntu repos do not provide a complete enough (static, with simulator)
riscv toolchain.
Instead of building it manually, we first install nix
(https://nixos.org/nix) and then use that to install a cross gcc, spike,
and the proxy kernel.

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This is a bit unorthodox, but it's the most reliable (and quite fast, since there should be no compiling) way I found to have gcc, spike, and pk in the ubuntu image.

I will shortly make another pr with the testing.py patch.

[talsewell]

Quick comment: I don't think we're going to change the base image at this point in the cycle. There are some tricky compatibility issues with some of the tools some people are using, and there isn't a lot of time to check everything. Is that necessary for the other parts to work?

Also, it's not clear to me that modifying the default docker image is the right way to go. It is good that we have a precise document for how to set up a system with the necessary tools. Do you have a measure of how much additional time/space is needed to build the image with an additional nixos install in it? If it's anything much, we might want an alternative image instead, or just run these commands on one of our testing setups (we aren't strictly using the docker image for all testing).

[fgaz]

I don't think we're going to change the base image at this point in the cycle. There are some tricky compatibility issues with some of the tools some people are using, and there isn't a lot of time to check everything

Makes sense, I didn't think about that.

Is that necessary for the other parts to work?

I don't think it is. I'm rebuilding the image on bionic right now, and I'll report back as soon as it finishes.

Do you have a measure of how much additional time/space is needed to build the image with an additional nix install in it?

I do not have a specific number, but most of it should be downloaded from a binary cache, so it shouldn't take that much.

we might want an alternative image instead

If we take that path here's an image with the nix package manager already installed: https://hub.docker.com/r/nixos/nix/

or just run these commands on one of our testing setups

They should work on any semi-recent system, so that's doable too

[fgaz]

@talsewell Works fine on bionic

[oskarabrahamsson]

I included @fgaz changes into its own Docker image building on the base one. See #14.

Closing this.