LCOW: Auto-select OS

Signed-off-by: John Howard <jhoward@microsoft.com>

Addresses https://github.com/moby/moby/pull/35089#issuecomment-367802698.
This change enables the daemon to automatically select an image under LCOW
that can be used if the API doesn't specify an explicit platform.

For example:

FROM supertest2014/nyan
ADD Dockerfile /

And docker build . will download the linux image (not a multi-manifest image)

And similarly docker pull ubuntu will match linux/amd64
Upstream-commit: 35193c0e7dc301e1d2f6ea96e0ce34ffd2d4b88d
Component: engine

components/engine/pkg/system/lcow.go

@@ -59,10 +59,10 @@ func ParsePlatform(in string) *specs.Platform {
 
 // IsOSSupported determines if an operating system is supported by the host
 func IsOSSupported(os string) bool {
-	if runtime.GOOS == os {
+	if strings.EqualFold(runtime.GOOS, os) {
 		return true
 	}
-	if LCOWSupported() && os == "linux" {
+	if LCOWSupported() && strings.EqualFold(os, "linux") {
 		return true
 	}
 	return false