Add container runlabel command

Execute the command as described by a container image.  The value of the label is processed
into a command by:

1. Ensuring the first argument of the command is podman.
2. Substituting any variables with those defined by the environment or otherwise.

If no label exists in the container image, nothing is done.

podman container runlabel LABEL IMAGE extra_args
Signed-off-by: baude <bbaude@redhat.com>

libpod/image/image.go

@@ -744,6 +744,20 @@ func (i *Image) Labels(ctx context.Context) (map[string]string, error) {
 	return imgInspect.Labels, nil
 }
 
+// GetLabel Returns a case-insensitive match of a given label
+func (i *Image) GetLabel(ctx context.Context, label string) (string, error) {
+	imageLabels, err := i.Labels(ctx)
+	if err != nil {
+		return "", err
+	}
+	for k, v := range imageLabels {
+		if strings.ToLower(k) == strings.ToLower(label) {
+			return v, nil
+		}
+	}
+	return "", nil
+}
+
 // Annotations returns the annotations of an image
 func (i *Image) Annotations(ctx context.Context) (map[string]string, error) {
 	manifest, manifestType, err := i.Manifest(ctx)