Expose OS platform

[zerok]

It should be possible to differentiate between different operating systems inside a template file. My main motivation for this is that Docker for Mac cannot reliably access the host's external IP address but relies on a special hostname (docker.for.mac.localhost) which is usually mapped to 192.168.65.1.

Inside a docker-compose.yml I now want to be able to do something like this:

    extra_hosts:
      - "vault:{{ if eq .OS.Platform "darwin" }}192.168.65.1{{ else }}{{ .Network.ExternalIP }}{{ end }}"