platforms: provide simpler function for common use

Signed-off-by: Stephen J Day <stephen.day@docker.com>

platforms/defaults.go

@@ -6,8 +6,13 @@ import (
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
-// Default returns the current platform's default platform specification.
-func Default() specs.Platform {
+// Default returns the default specifier for the platform.
+func Default() string {
+	return Format(DefaultSpec())
+}
+
+// DefaultSpec returns the current platform's default platform specification.
+func DefaultSpec() specs.Platform {
 	return specs.Platform{
 		OS:           runtime.GOOS,
 		Architecture: runtime.GOARCH,