According to some messages on Hotio’s Discord server from 2023-11-25, qBitTorrent moved from fixed admin credentials to randomized at initialization. I think MrHotio’s message about that crypto miner was likely a joke about people installing the older vulnerable version and the efficiency of unauthorized people installing xrig on servers with default credentials.
If author was pinned to an old version of the docker image and their server had internet-visible IP, they probably got their server infected because of weak security defaults in the app installed on the image.
Edit: Scion9066’s comment shows that dBitTorrent’s previous release version patches multiple security bugs, so vulnerabilities might apply to all versions older than about 1 week, not my guess of 2 years.