fix: Tag using image creation timestamp

Tags should not depend on the date and time the image is published but
on when it was created.

publish.sh

@@ -8,7 +8,7 @@ SOURCE_IMAGE=$SOURCE_REGISTRY/$1:$DEVUAN_CODENAME$BUILD_VARIANT-$BUILD_ID
 TARGET_IMAGE=$TARGET_REGISTRY/$1
 
 docker pull --quiet "$SOURCE_IMAGE"
-TIMESTAMP=$(date -u +%F)
+TIMESTAMP=$(docker inspect --format='{{.Created}}' "$SOURCE_IMAGE" | sed 's/T.*//')
 
 docker tag "$SOURCE_IMAGE" \
        "$TARGET_IMAGE:$DEVUAN_CODENAME$BUILD_VARIANT-$TIMESTAMP"