Allow configuration of network namespace container "extra_hosts"

[djenriquez]

When using the pause container as the parent namespace container for tasks to join their network on, it becomes important to have the ability to control the /etc/hosts file so that services can continue to use a hostname as the connection string for their upstreams. This can be done by configuring "extra_hosts" for the docker driver, but we don't seem to have the ability to control the configuration of the pause container.

Since other tasks join the pause container's network, they are not allowed to use "extra_hosts" as they get the error "Conflicting options: custom host-to-IP mapping and the network mode.".

[tyler-domitrovich]

Similarly, it would be nice to have control over the /etc/resolve.conf file so that services may have control over the nameservers they evaluate. This can be done by configuring the "dns", "dns-opt", and "dns-search" options for the docker driver.

Since other tasks join the pause container's network, they are unable to use those options and receive the following error: "conflicting options: dns and the network mode"

[tgross]

#10766 will provide a better workaround for this issue but won't quite give full control, so I'm not going to close this issue out when that merges.