Add image store

The image store abstracts image handling. It keeps track of the
available images, and makes it possible to delete existing images or
register new ones. The image store holds references to the underlying
layers for each image.

The image/v1 package provides compatibility functions for interoperating
with older (non-content-addressable) image structures.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

image/rootfs_unix.go

@@ -0,0 +1,23 @@
+// +build !windows
+
+package image
+
+import "github.com/docker/docker/layer"
+
+// RootFS describes images root filesystem
+// This is currently a placeholder that only supports layers. In the future
+// this can be made into a interface that supports different implementaions.
+type RootFS struct {
+	Type    string         `json:"type"`
+	DiffIDs []layer.DiffID `json:"diff_ids,omitempty"`
+}
+
+// ChainID returns the ChainID for the top layer in RootFS.
+func (r *RootFS) ChainID() layer.ChainID {
+	return layer.CreateChainID(r.DiffIDs)
+}
+
+// NewRootFS returns empty RootFS struct
+func NewRootFS() *RootFS {
+	return &RootFS{Type: "layers"}
+}