GitHub Action: initial DockerHub registry push logic

[jefferya]

Initial attempt at adding a GitHub Action that mirrors DockerHub auto build

Setup

• DockerHub: create access token - in "my account" --> "security" -> "new access token"
• GitHub: for each repository in "settings" -> "secrets" - "new repository secret" add the following two secrets
  • DOCKERHUB_TOKEN
  • DOCKERHUB_USERNAME

Once PR is merged into the default ("master" in this case) branch, the "actions" tab in GitHub should show the progress

Note: this initial likely doesn't match the tagging practice

References:

• https://github.com/docker/metadata-action#typeref
• https://docs.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions#github-context
• https://docs.github.com/en/actions/guides/publishing-docker-images
• https://docs.github.com/en/actions/reference/encrypted-secrets
• https://docs.docker.com/docker-hub/access-tokens/