Add default value of the MERCURE_TRANSPORT_URL in Dockerfile

This should return the path that was changed in #766.

Author: 7-zete-7

Dockerfile

@@ -36,6 +36,9 @@ RUN set -eux; \
 # https://getcomposer.org/doc/03-cli.md#composer-allow-superuser
 ENV COMPOSER_ALLOW_SUPERUSER=1
 
+# Transport to use by Mercure (default to Bolt)
+ENV MERCURE_TRANSPORT_URL=bolt:///data/mercure.db
+
 ENV PHP_INI_SCAN_DIR=":$PHP_INI_DIR/app.conf.d"
 
 ###> recipes ###