Console tracer output goes to stderr, so docker logs is the right place to look. The tracer is configuration just like the milter, though, and saving it changes nothing until you run Reload server settings or restart, which would explain why nothing showed up at all.
After the reload, set the tracer to info with no event filter for one test and send a message. A milter.action-accept line means the milter ran. No milter lines at all means it isn’t loaded, and the reload result will say why; the likely cause is in Stalwart 0.16.22 accepts SMTP DATA without a Rspamd Milter scan.