add Dockerfile

[aphex3k]

This allows for running nostril from a docker image instead of having to worry about building it first.

docker run --rm aphex3k/nostril:testing --envelope --dm <hex-pub> --sec <hex-priv> --content "test" | websocat wss://relay.damus.io

[SonnyAD]

Works good, thanks

[501st-alpha1]

I tried building this Docker image myself, and it failed on building docs, so I had to add scdoc to the apk add line. After that, it built successfully with:

docker build -f docker/Dockerfile -t nostril .

Is there anything else that is preventing this PR from being merged (aside from perhaps a rebase)?