Move to Github Actions for Release CI, publish to Dockerhub and GHCR

[dereknola]

• Remove release steps from Drone CI. Note that PR CI still runs there and test builds.
• Arrived at the following setup for CI release:
  • All 3 architectures are build in parallel, pushing digest releases to dockerhub and GHCR
  • At the end, a merge-manifests job combines them into the actual tagged release and pushed to registries
  • amd64 is built on amd64 host
  • arm64 is built on arm64 host
  • arm32 is emulated on arm64 host (way better performance than emulating on amd64 host)
• latest tags are now published.
• VERSION-ARCH tags are no longer published due to limitations on docker push action. So no more v0.4.10-arm tag, just the combined tag of v0.4.10.

Note the following must be added before this CI can work

• Action secret DOCKER_USER .... user account (for k3s-io this will be rancher, due to legacy publishing)
• Action secret DOCKER_TOKEN dockerhub api token with "read/write" permissions