Set the images' own OCI labels instead of Ubuntu's (closes #216)

The image sets title, description, version, created, revision and
source; the last three known only at build time come from build
arguments that default to unknown, and script/docker and
script/cibuild pass them. The example app sets its own title and
description.

Model: opus-5-5

deploy/example-app/Dockerfile

@@ -9,6 +9,10 @@
 ARG SWW_IMAGE=smallwebwaf
 FROM ${SWW_IMAGE}
 
+# Its own title and description, in place of the smallwebwaf image's.
+LABEL org.opencontainers.image.title="smallwebwaf-example-app" \
+    org.opencontainers.image.description="An example app built on the smallwebwaf image."
+
 # Packages the app needs, from the nixpkgs in the image.
 RUN nix-env -iA nixpkgs.busybox