feat: Simplify unify containers, but with artifacts [WIP]

[daler]

#959 uses ghcr.io as a cache mechanism. However, images stored there are not straightforward to clean up. Artifacts, on the other hand, expire after 90 days. This PR is an experiment to see if it will simplify things further to use artifacts rather than ghcr.io.

[daler]

Notes so far:

• Artifacts are created out of containers created during each job and then downloaded if/when needed in subsequent jobs. podman load <tarball> then is run on the downloaded artifacts. This seems straightforward, and I think this is a little more transparent than the previous "ghcr.io as a cache" strategy
• I removed caching and dependencies. Every job runs every time. This is terribly inefficient for debugging! But it does reduce complexity, which is hopefully more important for maintainability over the long term.
• Images are created in podman but are used in docker by bioconda-utils. Previously, pushing to ghcr.io made this simpler because both docker and podman could push/pull to the registry in a runtime-agnostic manner. Now, passing artifacts around makes things more annoying especially with respect to manifests.
• Apparently manifests cannot be used with a local docker runtime and MUST be pushed to a registry for docker to use them. So this PR adds a local docker registry for the CI running on localhost:5000.
• Where things currently break is in the manifest creation. The idea is for buildah to build a manifest out of the existing archs and push it to the local docker registry at localhost:5000, and then get bioconda-utils to use that registry for the tests.

[daler]

In good shape; closing in favor of the cleaned-up #1049.