Assign iface parameter from defined variable

Since `$CONTAINER_IFNAME ` is already defined in weave, it would be better to use it to assign `iface` parameter in two Dockerfiles.
Not that we have to do it here, but I'd suggest that this line is getting long and breaking it up into `$DOCKER_RUN_ARGS` and `$WEAVER_ARGS` might be a good idea.