Override _mount_binds in VendorDockerVM: for GNS3_SKIP_INIT containers the
/etc/network volume (GNS3's own network config consumed by init.sh's ifup)
is dead weight — init.sh never runs and the NOS manages its own
interfaces. The override removes the bind, filters /etc/network out of
self._volumes (keeping GNS3_VOLUMES and the bridge/fix passes consistent)
and deletes the host-side skeleton directory created by the base class.
Without GNS3_SKIP_INIT the mount is kept, matching base behaviour.
_persistent_volumes() is removed — the mount override is now the single
filter point.