Add field to Container for client-defined data

This field allows a client to store specialized information in the
container metadata rather than having to store this itself and keep
the data in sync with containerd.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

container.go

@@ -12,6 +12,7 @@ import (
 	"github.com/containerd/containerd/containers"
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/typeurl"
+	prototypes "github.com/gogo/protobuf/types"
 	specs "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/pkg/errors"
 )
@@ -42,6 +43,8 @@ type Container interface {
 	Labels(context.Context) (map[string]string, error)
 	// SetLabels sets the provided labels for the container and returns the final label set
 	SetLabels(context.Context, map[string]string) (map[string]string, error)
+	// Extensions returns the extensions set on the container
+	Extensions() []prototypes.Any
 }
 
 func containerFromRecord(client *Client, c containers.Container) *container {
@@ -158,6 +161,12 @@ func (c *container) Image(ctx context.Context) (Image, error) {
 	}, nil
 }
 
+func (c *container) Extensions() []prototypes.Any {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.c.Extensions
+}
+
 func (c *container) NewTask(ctx context.Context, ioCreate IOCreation, opts ...NewTaskOpts) (Task, error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()