Disassociate instance id from application

This moves the instance id out of the app so that it is associated with an
instantiation of the runtime. The instance id is stored on the background
context. This allows allow contexts using the main background context to
include an instance id for log messages. It also simplifies the application
slightly.

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

context/context.go

@@ -1,6 +1,7 @@
 package context
 
 import (
+	"code.google.com/p/go-uuid/uuid"
 	"golang.org/x/net/context"
 )
 
@@ -9,9 +10,31 @@ type Context interface {
 	context.Context
 }
 
-// Background returns a non-nil, empty Context.
+// instanceContext is a context that provides only an instance id. It is
+// provided as the main background context.
+type instanceContext struct {
+	Context
+	id string // id of context, logged as "instance.id"
+}
+
+func (ic *instanceContext) Value(key interface{}) interface{} {
+	if key == "instance.id" {
+		return ic.id
+	}
+
+	return ic.Context.Value(key)
+}
+
+var background = &instanceContext{
+	Context: context.Background(),
+	id:      uuid.New(),
+}
+
+// Background returns a non-nil, empty Context. The background context
+// provides a single key, "instance.id" that is globally unique to the
+// process.
 func Background() Context {
-	return context.Background()
+	return background
 }
 
 // WithValue returns a copy of parent in which the value associated with key is