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>

metadata/namespaces.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"github.com/boltdb/bolt"
+	"github.com/containerd/containerd/identifiers"
 	"github.com/containerd/containerd/namespaces"
 )
 
@@ -21,7 +22,7 @@ func (s *namespaceStore) Create(ctx context.Context, namespace string, labels ma
 		return err
 	}
 
-	if err := namespaces.Validate(namespace); err != nil {
+	if err := identifiers.Validate(namespace); err != nil {
 		return err
 	}