set the default value for POSTGRES_USER to postgres

This is done in order to ensure backward compatibility

Dockerfile

@@ -8,6 +8,7 @@ ENV WALLABAG_VERSION=2.0.7 \
     SYMFONY__ENV__DATABASE_NAME=symfony \
     SYMFONY__ENV__DATABASE_USER=root \
     SYMFONY__ENV__DATABASE_PASSWORD=~ \
+    POSTGRES_USER=postgres \
     SYMFONY__ENV__SECRET=ovmpmAWXRCabNlMgzlzFXDYmCFfzGv \
     SYMFONY__ENV__MAILER_HOST=127.0.0.1 \
     SYMFONY__ENV__MAILER_USER=~ \