buildRigNetworkLabels function
The labels rig puts on a network it creates.
Only the marker: a network is not hashed or reused the way a container
is, so it carries none of the identifying labels buildRigLabels adds.
rig prune needs just enough to recognise the network as rig's own.
Implementation
Map<String, String> buildRigNetworkLabels() => const {rigMarkerLabel: '1'};