Stage OCI container images through GitHub Actions (GHA) to GitHub Container Registry (GHCR) / Provide OCI images for ARM64

[amotl]

Even if GH-66 is not strictly needed, as Docker Inc. will continue their FOSS offering, this patch would set the stage to do so without much ado.

Its main purpose, however, is to resolve GH-68, and start providing OCI images for ARMv7 and ARM64, other than AMD64. We are using this recipe on a few other repositories, including mqttwarn, and it works well.

As a benefit in comparison to the previous build system, this recipe will provide image variants for latest, nightly, and pr-XXX builds.