identifiers: use common package for identifier validation

A few days ago, we added validation for namespaces. We've decided to
expand these naming rules to include containers. To facilitate this, a
common package `identifiers` now provides a common validation area.
These rules will be extended to apply to task identifiers, snapshot keys
and other areas where user-provided identifiers may be used.

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

namespaces/context.go

@@ -3,6 +3,7 @@ package namespaces
 import (
 	"os"
 
+	"github.com/containerd/containerd/identifiers"
 	"github.com/pkg/errors"
 	"golang.org/x/net/context"
 )
@@ -61,8 +62,8 @@ func NamespaceRequired(ctx context.Context) (string, error) {
 		return "", errNamespaceRequired
 	}
 
-	if err := Validate(namespace); err != nil {
-		return "", err
+	if err := identifiers.Validate(namespace); err != nil {
+		return "", errors.Wrap(err, "namespace validation")
 	}
 
 	return namespace, nil