Expose MAX_PARALLEL_PSQL to users (#927)

Allow users to override how many import-sql files are loaded in parallel at the same time.

This change should be a noop

docker-compose.yml

@@ -40,6 +40,8 @@ services:
       BORDERS_CLEANUP_FILE: ${BORDERS_CLEANUP_FILE}
       BORDERS_PBF_FILE: ${BORDERS_PBF_FILE}
       BORDERS_CSV_FILE: ${BORDERS_CSV_FILE}
+      # Control import-sql processes
+      MAX_PARALLEL_PSQL: ${MAX_PARALLEL_PSQL}
     networks:
       - postgres_conn
     volumes: