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

script/cibuild

@@ -20,8 +20,13 @@ main() {
     # the version a build stage derives from the .git in the context.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
+    revision="$(git rev-parse HEAD 2>/dev/null || true)"
+    [ -n "$revision" ] || revision="unknown"
+    created="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
     docker build --no-cache \
         --build-arg VERSION="$version" \
+        --build-arg REVISION="$revision" \
+        --build-arg CREATED="$created" \
         -t "$("$SCRIPT_DIR/projectname")" .
 }