Added importmap compile step in production build if needed

This is usefull for plug-and-play production mode for asset-mapper

Author: florentdestremau

Dockerfile

@@ -92,4 +92,7 @@ RUN set -eux; \
 	composer dump-autoload --classmap-authoritative --no-dev; \
 	composer dump-env prod; \
 	composer run-script --no-dev post-install-cmd; \
+	if [ -f importmap.php ]; then \
+		php bin/console asset-map:compile; \
+	fi; \
 	chmod +x bin/console; sync;