From the container console start the nat.sh script from /data directory Configure /data/nat.sh according to your topology, then start it start dnsmasq for DHCP and iptables for NAT (masquerade)
sh /data/nat.sh
Example of deployment:
GNS3 + Docker: Internet modem container
https://github.com/GNS3/gns3-registry/tree/master/docker/dhcp