Initial pull implemented.

Support for proxy repositories by pulling from the proxy and then tagging the image.

pkg/runtime/container_runtime.go

@@ -0,0 +1,12 @@
+package runtime
+
+// ContainerRuntime defines the interface for managing containerd images
+type ContainerRuntime interface {
+	// List images: returns map of container image to whether or not it is pinned
+	List() (map[string]bool, error)
+	Pin(imageRef string) error
+	Unpin(imageRef string) error
+	Pull(imageRef string) error
+	Tag(imageFrom, imageTo string) error
+	Remove(imageRef string) error
+}