First naked-singularity containers hosted on GitHub Container Registry

Successfully tested by pushing containers [1] built on Jetstream to
GitHub Container Registry (GHCR), pulling containers [2] hosted on
GHCR to Expanse, and then running osu_bw and osu_latency benchmarks.

[1]

export GH_PAT='<GITHUB_PERSONAL_ACCESS_TOKEN>'
singularity push --docker-login mkandes --docker-password "${GH_PAT}" \
  omb-5.6.3-ubuntu-18.04-openmpi-3.1.6.sif \
  oras://ghcr.io/mkandes/naked-singularity:omb-5.6.3-ubuntu-18.04-openmpi-3.1.6

[2]

singularity pull oras://ghcr.io/mkandes/naked-singularity:omb-5.6.3-ubuntu-18.04-openmpi-3.1.6