Taming dnsmasq and Docker containers, an Ubuntu 16.04 LTS story
Foreword, by default Docker daemon pass to container DNS from /etc/resolv.conf file from the host machine, except local addresses. Ubuntu 16.04 LTS Network Manager use dnsmasq by default, so in the Docker host /etc/resolv.conf there is only a 127.0.0.1 entry. In this configuration Docker daemon pass to container Google public DNS by default :)
If you working with VPN, custom DNS configuration or simply you change host machine DNS after container startup you got in trouble if your containers needs those DNS.
Summary In networks where external DNS servers are blocked, Docker containers running on Ubuntu hosts can't resolve DNS…github.com
/etc/init.d/network-manager stop && /etc/init.d/network-manager start
after that you can monitor every dnsmasq resolution from /var/log/syslog