Remove config folder from pre-create

The folder doesn't exist in our CI at the moment since we use the
default config for testing rather than an optimized config. This means
that the start command doesn't work if it's included.

solr/Dockerfile

@@ -2,4 +2,4 @@ ARG VERSION=latest
 
 FROM solr:$VERSION
 
-ENTRYPOINT ["solr-precreate", "drupal", "/solr-conf"]
+ENTRYPOINT ["solr-precreate", "drupal"]