Add dockerhub publish workflow

Created by: lwj5

Hi @wing328

I'm not too sure about how your release process works. But if you are keen to have a GitHub action based publish for docker image here's a working sample.

Here's

The action log: https://github.com/lwj5/openapi-generator/runs/3269899575?check_suite_focus=true Ignore the action failure, there's a temp 503 issue storing image cache to Github cache post-release

The published artefact. Both amd64 and arm64 included, you can add more if you want. https://hub.docker.com/r/lwj5/openapi-generator-cli/tags?page=1&ordering=last_updated

Currently, it is sending to my registry account. Will need to understand more about your requirements, and make changes accordingly.

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