feat: add dockerfile

[rare-magma]

Containerizing intermodal makes it easier to use without installing it.

The docker image can be hosted on ghcr.io and its build automated via github action: https://docs.github.com/en/actions/publishing-packages/publishing-docker-images

The container can then be used like this:

docker run --rm --init --interactive --tty --volume "$(pwd)":/mnt ghcr.io/casey/intermodal torrent show /mnt/example.torrent

[rare-magma]

@casey I've simplified the dockerfile to use the official docker library rust image as the build stage. The drawback with this approach is that any change in the repository invalidates the docker cache and results in a full rebuild. In the end it shouldn't matter if the image is only built once per release.