Add Docker image

[RyanGlScott]

Fixes #71.

[RyanGlScott]

I have verified that this image builds locally and runs as you would expect it to. Some things left to do:

• This image does not build our forked versions of the Rust standard libraries (currently housed here). After some thought, I think this is fine. The plan in Maintain modified Rust standard libraries and translate_libs.sh in a separate repo crucible#1252 is to move this code to its own repo, and once we do that, we can provide a separate Docker image that includes the compiled versions of the forked standard libraries.
• We should build this image as part of CI and push it to the GitHub Container Registry (ghcr.io). For now, I think it will suffice to do this on each commit to the master branch (where the image is tagged with nightly), as well as on each time a git tag is pushed (where the image is tagged with the name of the git tag).
• We should advertise this image in the mir-json README.

[spernsteiner]

This image does not build our forked versions of the Rust standard libraries

Is mir-json usable for anything without those libraries? Even #[no_std] code is going to need libcore, and using the standard libcore (e.g. cargo saw-build -Zbuild-std=core) would be pretty severely broken (IIRC even some basic slice operations are patched in our fork).

[RyanGlScott]

Is mir-json usable for anything without those libraries?

No, not really. (And especially so after #67 is fixed.)

I suppose there is a question of whether we want the mir-json repo to house a dev image that provides nothing but mir-json, for which the repo that houses the fork of the Rust standard libraries could use to build its own image. Alternatively, we could not offer an image in the mir-json repo at all and instead provide an image in the other repo that includes all of mir-json + the forked standard libraries. I don't have a particularly strong opinion on how we do it.

Yet another alternative would be to move the forked standard libraries into this repo so that both them and mir-json are all under the same repo.

[spernsteiner]

I have no real preference, except that I don't think we should advertise an image to users that they can't actually use for anything

[RyanGlScott]

I have rebased this on top of the changes from GaloisInc/crucible#1252, and I have updated the Docker image to include the translated standard libraries. Next up: a CI job.

[qsctr]

I am not a CI expert but this looks ok to me