Adds a new workflow for building the image using GitHub Actions

[stumpylog]

This adds a GitHub Actions workflow to build and optionally push the Docker image, using the standard actions, mostly from Docker themselves.

Features

• Pushes only on the push of a vX.Y.Z tag, builds on pushes to main or feature/* branches
• Automatic standard labels for the image
• Automatic image tags including allowing users to follow a release series (vX.Y vs vX.Y.Z)
• Same multi-arch building
• No need for build-multiarch.sh anymore, though I kept it for reference