context: remove definition of Context

Back in the before time, the best practices surrounding usage of Context
weren't quite worked out. We defined our own type to make usage easier.
As this packaged was used elsewhere, it make it more and more
challenging to integrate with the forked `Context` type. Now that it is
available in the standard library, we can just use that one directly.

To make usage more consistent, we now use `dcontext` when referring to
the distribution context package.

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

context/context.go

@@ -1,21 +1,16 @@
 package context
 
 import (
+	"context"
 	"sync"
 
 	"github.com/docker/distribution/uuid"
-	"golang.org/x/net/context"
 )
 
-// Context is a copy of Context from the golang.org/x/net/context package.
-type Context interface {
-	context.Context
-}
-
 // instanceContext is a context that provides only an instance id. It is
 // provided as the main background context.
 type instanceContext struct {
-	Context
+	context.Context
 	id   string    // id of context, logged as "instance.id"
 	once sync.Once // once protect generation of the id
 }
@@ -42,17 +37,10 @@ var background = &instanceContext{
 // 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 {
+func Background() context.Context {
 	return background
 }
 
-// WithValue returns a copy of parent in which the value associated with key is
-// val. Use context Values only for request-scoped data that transits processes
-// and APIs, not for passing optional parameters to functions.
-func WithValue(parent Context, key, val interface{}) Context {
-	return context.WithValue(parent, key, val)
-}
-
 // stringMapContext is a simple context implementation that checks a map for a
 // key, falling back to a parent if not present.
 type stringMapContext struct {