Add Docker image publishing via CI

[jhelison]

Is your feature request related to a problem? Please describe.
No automated Docker release; users build their own images manually.

Describe the solution you'd like
On tagged releases, have CI build and push Docker image (e.g. kiichain:3.0.0) to Docker Hub or GitHub Container Registry.

Describe alternatives you've considered
Maintainers manually upload images.

Additional context
Requires Dockerfile review and credentials stored in GitHub secrets.

[mrmintall]

I would like to work on this issue