osbuild: add new org.osbuild.skopeo stage

Add bindings for the `org.osbuild.skopeo` that can be used to copy
container images, accessed via the `org.osbuild.containers` input,
into images.
The constructor is designed with ease of use in mind and takes
the needed container inputs and the storage path option, i.e.
where to store the container in the images.

internal/osbuild/skopeo_stage.go

@@ -0,0 +1,31 @@
+package osbuild
+
+type SkopeoDestination struct {
+	Type          string `json:"type"`
+	StoragePath   string `json:"storage-path,omitempty"`
+	StorageDriver string `json:"sotrage-driver,omitempty"`
+}
+
+type SkopeoStageOptions struct {
+	Destination SkopeoDestination `json:"destination"`
+}
+
+func (o SkopeoStageOptions) isStageOptions() {}
+
+func NewSkopeoStage(images ContainersInput, path string) *Stage {
+
+	inputs := ContainersInputs{
+		"images": images,
+	}
+
+	return &Stage{
+		Type: "org.osbuild.skopeo",
+		Options: &SkopeoStageOptions{
+			Destination: SkopeoDestination{
+				Type:        "containers-storage",
+				StoragePath: path,
+			},
+		},
+		Inputs: inputs,
+	}
+}