add GitHub container registry mirror

[gdams]

Given the number of container users we have, it makes sense to provide a backup registry incase there is a dockerhub outage. This PR adds a GH workflow that mirrors whatever is in the upstream official image

I ran this workflow in my fork to give people an idea of what this will look like: https://github.com/gdams/containers/pkgs/container/eclipse-temurin

[karianna]

Should add docs in our README or infra or somewhere

[gdams]

I wonder whether it might make sense to build these using bashbrew in GitHub actions rather than mirroring everything from Dockerhub. I don't know whether bashbrew can build all the arches or whether we'd need a machine for each architecture that we support. CC @tianon if you wouldn't mind pointing me in the right direction here. I did come across https://github.com/docker-library/meta/blob/subset/.github/workflows/build.yml which seems to be nearly what I'm looking for.