Added automated building of Docker image

[mtak]

This PR adds automated builds of the Dockerfile in GitHub Actions and pushes the result towards Docker Hub.

All that's needed to enable this and make it work properly:

On Docker Hub

• Create a Docker Hub account
• Create an access token (Profile->Security->New Access Token)
• Create a repository named coredns-dockerdiscovery on Docker Hub

On GitHub:

• Set up the following variables in the repo settings:
  • DOCKERHUB_USERNAME - your Docker Hub username
  • DOCKERHUB_TOKEN - the access token

Example run can be seen at my fork

Note: normally I'd allow edits on my branch, but in this case it could give you access to my GitHub secrets. Feel free to clone and resubmit the PR if you feel it needs work and I'm not responding quickly enough to comments.

[u6bkep]

why not use the github container registry?

https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry