containerfile: warn if we don't have a Dockerfile

Currently if no Dockerfile is specified or found, we exit later with
an obscure error.  Check this after the element search; if we still
don't have something to build then we can't continue.

Change-Id: Ifb17a0995fab0ccfe7ee08363676c1fa57e37592

diskimage_builder/elements/containerfile/root.d/08-containerfile

@@ -57,6 +57,11 @@ if [ -z "${DIB_CONTAINERFILE_DOCKERFILE:-}" ]; then
     done
 
     $_xtrace
+
+    if [ -z "${DIB_CONTAINERFILE_DOCKERFILE:-}" ]; then
+        echo "*** DIB_CONTAINERFILE_DOCKERFILE not specified or found!"
+        exit 1
+    fi
 fi
 
 # Use the image cache directory as the default context, so anything