Add SOCI support

[ipmb]

SOCI images start faster on Fargate https://elasticscale.com/blog/reduce-aws-fargate-pull-times-with-soci/

Docker doesn't seem to support this, but nerdctl does. It may be worth switching to nerdctl completely. See finch for an example of how to integrate it into a different CLI.

It requires that the soci binary exists on the path and buildkitd is running.

Here is an end-to-end example in GitHub Actions: https://github.com/lincolnloop/lincolnloop/blob/soci-image/.github/workflows/soci_build.yml

[ipmb]

Played around with this a little here, but didn't get it working. It is really slow and errors out. Not immediately clear what's different about the environment vs. GHA.

In the Lincoln Loop AWS account: https://us-east-1.console.aws.amazon.com/codesuite/codebuild/135549385118/projects/soci-test