/_ping handler: return OSType http header

The docker client expects to read the OSType header from the `/_ping` response in order to determine the OS type of the server, for example, when running `docker run --device=/dev/fuse ...`

https://github.com/moby/moby/blob/master/client/ping.go#L57

Signed-off-by: chnrxn <cohawk@yahoo.com>

pkg/api/handlers/compat/ping.go

@@ -3,6 +3,7 @@ package compat
 import (
 	"fmt"
 	"net/http"
+	"runtime"
 
 	"github.com/containers/buildah"
 )
@@ -18,6 +19,7 @@ func Ping(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Builder-Version", "")
 	w.Header().Set("Docker-Experimental", "true")
 	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("OSType", runtime.GOOS)
 	w.Header().Set("Pragma", "no-cache")
 
 	w.Header().Set("Libpod-Buildah-Version", buildah.Version)

test/apiv2/01-basic.at

@@ -11,6 +11,7 @@ t GET  /libpod/_ping 200 OK
 t HEAD /libpod/_ping 200
 
 t GET  _ping        200 OK
+like "$(<$WORKDIR/curl.headers.out)" $'.*\nOstype: ' "#19767 - undocumented part of docker API"
 t HEAD _ping        200
 t GET  libpod/_ping 200 OK
 t HEAD libpod/_ping 200