@iesnaola: I presume this is a docker image built by yourself from a Virtuoso open source build, rather than using one of the official OpenLink Virtuoso Docker Images ?
Looking at your command it is missing the Virtuoso “isql” tool’s exec option to execute the Virtuoso commands/queries against it, see the isql documentation for usage information.
Thus using our Virtuoso open source docker, I can successfully execute the following:
De-iMac-2692:vos hwilliams$ docker exec -it vos /opt/virtuoso-opensource/bin/isql -U dba -P mysecret exec="ld_dir('/opt/virtuoso-opensource/vad/', 'myfile.rdf', 'http:/myGraph');"
OpenLink Virtuoso Interactive SQL (Virtuoso)
Version 07.20.3230 as of Sep 13 2019
Type HELP; for help and EXIT; to exit.
Connected to OpenLink Virtuoso
Driver: 07.20.3230 OpenLink Virtuoso ODBC Driver
Done. -- 0 msec.
De-iMac-2692:vos hwilliams$