Okay, that hinted in the right direction.
Indeed, the docker image in the launcher script was too old. We did not mess with the script though, and it was up to date with origin/master.
What we missed was that the main branch of the discourse repository is has been renamed to “main” (formerly “master”).
After pulling from the main branch, the launcher worked with our original configuration, all plugins and without the EMBER_CLI_PROD_ASSETS workaround.