Create a Docker image containing cargo-mutants

[sourcefrog]

• Mention it in the readme and manual
• Build linux/arm64 images: GitHub actions has arm64 workers now but it seems like we need to build both images separately and then merge them

[fence-sitter]

I have opened #625 and #626, for your perusal. Please let me know which approach you prefer and I will close the other.

The simple dockerfile (#625) essentially does cargo install cargo-mutants.

• Pros:
  • Allows you to specify (at build time) the Rust image to use as a base layer as well as the version of cargo-mutants to install. You can maintain debian, slim and alpine images with the one file. This is the case for both images now.
  • Very simple to understand and maintain.
• Cons:
  • Multi-platform images are harder/slower to create. I think docker buildkit emulates other platforms using QEMU?

The multi-platform dockerfile (#626) cross-compiles for the target architecture.

• Pros:
  • Multi-platform images are done.
• Cons:
  • Requires the use of cross-platform helper scripts. I'm not sure if you're against this or not.
  • Having debian, slim and alpine variants requires two dockerfiles (one for the Debian base and one for the Apline base). Not the case any more.
  • Can be intimidating if Docker isn't your forte.

Other notes:

• Neither image has nextest in it. We should probably bake it in, right? I've added nextest to the mulistage image, and it can easily be added to the simple image.
• I have no experience with GitHub actions (yet), so I haven't added anything to lint the dockerfile, build the image or push it to a registry. I'm in the process of getting this together.
• I have confirmed (using dive) that the bind/cache mounts are correct and that the built image does not include any leftover bits and bobs. The final image is larger than the base Rust image by ~10 MB, which is the size of the cargo-mutants binary`.
• Both dockerfiles have been run through hadolint. The multi-platform file passes with some hadolint ignore directives.