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>

plancontext/context.go

@@ -17,9 +17,7 @@ type Context struct {
 
 func New() *Context {
 	return &Context{
-		Platform: &platformContext{
-			platform: defaultPlatform,
-		},
+		Platform: &platformContext{},
 		FS: &fsContext{
 			store: make(map[string]*FS),
 		},