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),
 		},

plancontext/platform.go

@@ -5,28 +5,27 @@ import (
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
-var (
-	// Default platform.
-	// FIXME: This should be auto detected using buildkit
-	defaultPlatform = specs.Platform{
-		OS:           "linux",
-		Architecture: "amd64",
-	}
-)
-
 type platformContext struct {
-	platform specs.Platform
+	platform *specs.Platform
 }
 
 func (c *platformContext) Get() specs.Platform {
-	return c.platform
+	return *c.platform
 }
 
-func (c *platformContext) Set(platform string) error {
+func (c *platformContext) SetString(platform string) error {
 	p, err := platforms.Parse(platform)
 	if err != nil {
 		return err
 	}
-	c.platform = p
+	c.platform = &p
 	return nil
 }
+
+func (c *platformContext) Set(p specs.Platform) {
+	c.platform = &p
+}
+
+func (c *platformContext) IsSet() bool {
+	return c.platform != nil
+}