Automatically set target platform based on client architecture

Set the default platform based on the client's OS and architecture. This
function is the same one that buildkit uses (https://github.com/moby/buildkit/blob/master/frontend/dockerfile/builder/build.go#L100-L102) to set the default build target platform

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

plan/plan.go

@@ -123,7 +123,7 @@ func (p *Plan) configPlatform() error {
 	}
 
 	// Set platform to context
-	err = p.context.Platform.Set(platform)
+	err = p.context.Platform.SetString(platform)
 	if err != nil {
 		return err
 	}