Add containerd support to "--local" mode

Running `ko` with `--local` is super convenient 💯 Unfortunately it seems to works only with Docker daemons (correct me if I'm wrong).

I recently switched to `k3s` in my local setup, which does not rely on a Docker daemon. Instead, `k3s` exposes a `containerd` GRPC server at `/run/k3s/containerd/containerd.sock`.

What do you think about supporting publishing images directly to `containerd`, and enabling the feature via and environment variable, e.g. `KO_CONTAINERD_HOST`?

Start by tracing the --local mode entry point and its Docker-daemon publishing path, then inspect how the k3s containerd gRPC endpoint is configured. Done means an environment variable such as KO_CONTAINERD_HOST enables publishing directly to containerd while existing Docker-based --local behavior remains intact.

Written by the indexing model from the issue text.