Support alternative runtimes to docker in batch changes

Created by: eseliger

Today, src-cli and server-side batch changes both use docker to run the containers used to produce changes.

There is no direct mapping between a batch spec and docker, we don't allow to pass docker arguments or anything, so theoretically we could just swap out the backend with an alternative implementation like containerd, or nerdcli on top of it.

This would require crafting a generic interface that gets implemented by multiple runtimes.

Has been requested by https://github.com/sourcegraph/accounts/issues/5041.