Snapcraft cleanbuild hangs after "Network connection established"


I used to be able to use snapcraft cleanbuild on this computer, but now it just hangs after “Network connection established”:
$ snapcraft cleanbuild
Creating snapcraft-connivently-unexplodable-tu
Starting snapcraft-connivently-unexplodable-tu
Waiting for a network connection…
Network connection established

Any idea what I am doing wrong?

The snapcraft.yaml is here, if it helps:

Ubuntu Cosmic 18.10
snapcraft, version 3.1