ci: fix annoying docker warning

Docker currently warns that we are missing a default value.

Set this to scratch which will error if an appropriate image tag is not
passed in to silence the warning.

Author: willcl-ark

ci/test_imagefile

@@ -4,7 +4,8 @@
 
 # See ci/README.md for usage.
 
-ARG CI_IMAGE_NAME_TAG
+# We never want scratch, but default arg silences a Warning
+ARG CI_IMAGE_NAME_TAG=scratch
 FROM ${CI_IMAGE_NAME_TAG}
 
 ARG FILE_ENV