feat: add initial nilchain-devnet docker image

[mfontanini]

This adds the initial docker image which runs a preconfigured nilchain. A few notes:

• I trimmed the config files because most of their contents were the defaults.
• Parameters can be overridden via env vars (see readme).
• This uses by default a 1 second block time but can easily be overridden when running the container (see readme).
• This also adds CI jobs to build the image on push and build/push the image when a workflow is run on a tag. The push one was borrowed from the nilchaind one.

[andreasbros]

Nice!