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

Dockerfile

@@ -161,6 +161,20 @@ RUN git checkout --quiet --detach b4b2c785308b1ce785b6155c7fe5f16879080193 \
 # ubuntu 26.04, 2026-09-27
 FROM ubuntu@sha256:f144425ff09be612d6d9ad965196e9cdc23dae1f42110a8a11a3e9a8198759f7
 
+# The image's own labels, in place of the ones it would inherit from
+# Ubuntu's. script/docker and script/cibuild pass the build arguments;
+# a build that passes none labels the image unknown rather than leave
+# Ubuntu's values.
+ARG VERSION=unknown
+ARG CREATED=unknown
+ARG REVISION=unknown
+LABEL org.opencontainers.image.title="smallwebwaf" \
+    org.opencontainers.image.description="A web application firewall for self-hosted web apps: sww rproxy in front of an app, and sww waf, which decides." \
+    org.opencontainers.image.version="${VERSION}" \
+    org.opencontainers.image.created="${CREATED}" \
+    org.opencontainers.image.revision="${REVISION}" \
+    org.opencontainers.image.source="https://git.eeqj.de/sneak/smallwebwaf"
+
 # runit's install creates its _runit-log user with minsysusers, which
 # reads this file in place of runit's /usr/lib/sysusers.d/runit.conf.
 # runit's line leaves out the shell, and minsysusers prints a Perl