Set up RISC-V GNU docker repo

[Baltoli]

@Scott-Guest has figured out how to get the RISC-V GNU toolchain built and installed as a Docker image; it's a big image so we'll want to treat it similarly to the K / Z3 dependency and have a separate repo build and cache the images infrequently.

I've set up https://github.com/runtimeverification/riscv-gnu-toolchain-images for this purpose; we need @F-WRunTime to add the correct Dockerhub secrets, and @Scott-Guest to add the Dockerfile he's built. We can then compose a testing / release workflow.