And I think for windows you can use Teddybench, could have sworn it can endor mp3s.
The rpi is probably not suited for the task. I run “ancient” enterprise grade hardware, I allocated 4 cores à 2 threads with 16GB RAM and even that takes some time for smaller conversions. Given the CPU is older (Xeon 2600 V4 series), the better option is always to use your main machine, assuming it got more power than your machine running docker. For me it’s a 5800X, so plenty of power.