Add a Dockerfile to this repository

[jiphex]

This commit adds a two-stage Docker build to this repository. The first
step uses an Alpine Linux based image to build the source of jo, and
then this stage is discarded (except for the compiled binary) and a new
Alpine based image is produced containing the jo binary.

The built image can then be executed as:

[user@host] $ docker run --rm -ti jo a=b
{"a": "b"}

[jiphex]

This is useful for me right now, but once the Alpine package is live then it's probably unnecessary. 🤷‍

[jpmens]

Merged, thank you. I'm sure this is useful for some.