Automate docker.io images building

[glensc]

This uses https://github.com/docker/build-push-action to build images for linux/amd64,linux/arm64,linux/arm/v7 platforms.

Inspired by workflow file at:

• https://github.com/Taxel/PlexTraktSync/blob/0.15.0/.github/workflows/docker-image.yml

Resolves https://github.com/perftools/xhgui/issues/427

[glensc]

The workflow did not fire, it's because events created by actions don't fire other actions?

[glensc]

The problem seems to be described here:

• https://github.com/laminas/automatic-releases#triggering-release-workflow-events