DNS resolution fails on GCE VMs

[jdomeracki-coder]

Summary

DNS resolution fails on a vanilla GCE VM (related to #89, #108)

Expected behavior

• Boundary should support handling DNS resolution on GCE VMs and other Google Cloud workloads (eg. GKE Pods)
• Documentation should provide a guideline how to configure the above

Technical analysis

The metadata server's link-local IP 169.254.169.254 isn't routable from the dedicated network namespace

The solution implemented in #105 isn't sufficient since it only handles following default /etc/resolv.conf entries:
nameserver 127.0.0.53

Reference:
https://docs.cloud.google.com/compute/docs/troubleshooting/troubleshoot-metadata-server#failed-request