The server where you will run the script from, needs the SNMP tools and MIBs. You can get a good set of common MIBs when installing snmp-mibs-downloader on Debian ...
In this case snmpsim_data is a volume created under /var/lib/docker/volumes The filename/directory determines the SNMP community name. If you want to run snmpsimd with more flags then you can use ...