I encountered what I believe to be the same issue, in a non-Docker environment. After some digging, it became clear that the snmpget command that LibreNMS was generating looked like:
snmpget $FLAGS udp6:2001:db8::1234:161 $OIDS
instead of
snmpget $FLAGS udp6:[2001:db8::1234]:161 $OIDS
which would work fine for v4 addresses or DNS names, but not v6 literals.
I opened a PR to fix it: https://github.com/librenms/librenms/pull/13130