Document CONCOURSE_GARDEN_ENABLE_DNS_PROXY flag

[deniseyu]

This flag is configurable for Linux worker nodes, and is generally only used when running Concourse with Docker due to how DNS resolution works inside Docker containers when using embedded user-defined networks. Since this wouldn't be used by other deployment methods like BOSH or systemd, it doesn't make sense for this to live in the main docs but it could be useful to explain this flag in the Docker-specific docs.