Add support for 'image' volume driver

We added the concept of image volumes in 2.2.0, to support
inspecting an image from within a container. However, this is a
strictly read-only mount, with no modification allowed.

By contrast, the new `image` volume driver creates a c/storage
container as its underlying storage, so we have a read/write
layer. This, in and of itself, is not especially interesting, but
what it will enable in the future is. If we add a new command to
allow these image volumes to be committed, we can now distribute
volumes - and changes to them - via a standard OCI image registry
(which is rather new and quite exciting).

Future work in this area:
- Add support for `podman volume push` (commit volume changes and
  push resulting image to OCI registry).
- Add support for `podman volume pull` (currently, we require
  that the image a volume is created from be already pulled; it
  would be simpler if we had a dedicated command that did the
  pull and made a volume from it)
- Add support for scratch images (make an empty image on demand
  to use as the base of the volume)
- Add UOR support to `podman volume push` and
  `podman volume pull` to enable both with non-image volume
  drivers

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/volume.go

@@ -57,6 +57,15 @@ type VolumeConfig struct {
 	DisableQuota bool `json:"disableQuota,omitempty"`
 	// Timeout allows users to override the default driver timeout of 5 seconds
 	Timeout *uint `json:"timeout,omitempty"`
+	// StorageName is the name of the volume in c/storage. Only used for
+	// image volumes.
+	StorageName string `json:"storageName,omitempty"`
+	// StorageID is the ID of the volume in c/storage. Only used for image
+	// volumes.
+	StorageID string `json:"storageID,omitempty"`
+	// StorageImageID is the ID of the image the volume was based off of.
+	// Only used for image volumes.
+	StorageImageID string `json:"storageImageID,omitempty"`
 }
 
 // VolumeState holds the volume's mutable state.
@@ -149,7 +158,7 @@ func (v *Volume) MountCount() (uint, error) {
 
 // Internal-only helper for volume mountpoint
 func (v *Volume) mountPoint() string {
-	if v.UsesVolumeDriver() {
+	if v.UsesVolumeDriver() || v.config.Driver == define.VolumeDriverImage {
 		return v.state.MountPoint
 	}
 
@@ -250,6 +259,12 @@ func (v *Volume) IsDangling() (bool, error) {
 // drivers are pluggable backends for volumes that will manage the storage and
 // mounting.
 func (v *Volume) UsesVolumeDriver() bool {
+	if v.config.Driver == define.VolumeDriverImage {
+		if _, ok := v.runtime.config.Engine.VolumePlugins[v.config.Driver]; ok {
+			return true
+		}
+		return false
+	}
 	return !(v.config.Driver == define.VolumeDriverLocal || v.config.Driver == "")
 }