mount container runtime socket into steps

[frantjc]

GitHub Actions steps have access to the docker daemon, so we should, too

[frantjc]

the docker runtime now mounts its socket into any container it runs (assuming the docker client is connected to a unix socket--obviously we can't do anything if the daemon is on another server)

this doesn't scale up to k8s (unless the cluster is doing something dumb like using docker as the container runtime) or translate to containerd, but I don't really mind--other runtimes are part of sqnc's way of superseding GitHub Actions, so at that point I'll be okay with drift from "core" functionality