wkdev-update: Delete container before recreating

[ptomato]

I recently ran out of disk space while running wkdev-update and had to manually delete the existing container image with podman rmi before creating a new one. The containers are tens of gigabytes, so it'd be good for wkdev-update to first delete the old container.

(Alternatively, if we don't want to delete first by default, we could instead wait until the process fails due to disk space, then delete the old container and retry)