Issues with I2S Adafruit Amp

[Esser50K]

I'm trying to get this image working with the Adafruit I2S amp -> https://learn.adafruit.com/adafruit-max98357-i2s-class-d-mono-amp

Supposedly it is supported as mentioned here: https://github.com/balenalabs/balena-sound/blob/master/docs/05-audio-interfaces.md#dtoverlay-values

I've managed to get it working on the host machine. (I'm using RaspbianLite and not balenaOS btw)

When I run this image for raspberrypi3 (docker run --privileged -d -p 4317:4317 balenablocks/audio:raspberrypi3) and exec inside it I can run the alsa debug commands:

aplay -l:

**** List of PLAYBACK Hardware Devices ****
card 0: dac [snd_rpi_hifiberry_dac], device 0: HifiBerry DAC HiFi pcm5102a-hifi-0 [HifiBerry DAC HiFi pcm5102a-hifi-0]
  Subdevices: 0/1
  Subdevice #0: subdevice #0

So it seems to recognize the speaker. However running the speaker-test command produces no sound (although the command runs successfully). The command I use is speaker-test -c2 --test=wav -w /usr/share/sounds/alsa/Front_Center.wav. The same command works (e.g. produces sound) if I run it outside the container (although I have to run it with sudo).

What can I try to make this work? Do I need to use the balenaOS host image?