Github workflow to build and push Docker images on tag

[nevets963]

This PR adds a GitHub workflow to automatically build amd64 and arm64 Docker images and push them to the Docker registry (with a tag, and to also push/update the latest image tag).

This PR is largely based on this: https://github.com/getumbrel/umbrel-manager/blob/fc823490591ea55e26734d144a0527fe9618166d/.github/workflows/on-tag.yml

You need to create the necessary Github secrets (for the Docker registry credentials): https://docs.github.com/en/actions/security-guides/encrypted-secrets#creating-encrypted-secrets-for-a-repository

These need to be created:

• DOCKER_USERNAME
• DOCKER_PASSWORD
• DOCKER_HUB_USER

You can create a tag easily by following these instructions: https://stackoverflow.com/a/18223354 The workflow is setup to look for tags using semver. versioning with v as a prefix. i.e. v1.0.0 works, 1.0.0 doesn't.

[monlovesmango]

its working! thank you so much for creating this pr, would have taken me ages to figure out.

[monlovesmango]

this is going to make my deployments much easier, can't thank you enough