api, metadata: use map type for Container.Extensions

To allow for updating extensions without collisions, we have moved to
using a map type that can be explicitly selected via the field path for
updates. This ensures that multiple parties can operate on their
extensions without stepping on each other's toes or incurring an
inordinate number of round trips.

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

container_opts.go

@@ -135,13 +135,20 @@ func setSnapshotterIfEmpty(c *containers.Container) {
 // Make sure to register the type of `extension` in the typeurl package via
 // `typeurl.Register` otherwise the type data will be inferred, including how
 // to encode and decode the object.
-func WithContainerExtension(extension interface{}) NewContainerOpts {
+func WithContainerExtension(name string, extension interface{}) NewContainerOpts {
 	return func(ctx context.Context, client *Client, c *containers.Container) error {
 		any, err := typeurl.MarshalAny(extension)
 		if err != nil {
 			return err
 		}
-		c.Extensions = append(c.Extensions, *any)
+
+		if name == "" {
+			return errors.Wrapf(errdefs.ErrInvalidArgument, "extension key must not be zero-length")
+		}
+		if c.Extensions == nil {
+			c.Extensions = make(map[string]types.Any)
+		}
+		c.Extensions[name] = *any
 		return nil
 	}
 }