Add docker publishing to goreleaser config

[mattstratton]

Once we have a docker image that works, we want to have goreleaser publish it as part of the release process.

See https://goreleaser.com/customization/#Docker for details.

[mattstratton]

Possibly helpful blog post - https://carlosbecker.com/posts/goreleaser-docker/

GoReleaser Docker support - Carlos Alexandro Becker

The next GoReleaser version will have a more flexible Docker configuration format. In this post we’ll explore it a bit.