raspotify: Unable to get `mixer=alsa` working

Hello!

I cannot seem to get --mixer=alsa working.

I’m using the derkades/raspotify image with a modified entrypoint script. This does works without this parameter.

$ /usr/bin/librespot --name=Speakers --mixer=alsa --backend=alsa --enable-volume-normalisation --normalisation-pregain=3 --disable-discovery --disable-audio-cache --bitrate=320 --initial-volume=70 --username=xxxxx --password=xxxxx
[2021-11-09T10:44:44Z INFO  librespot] librespot (raspotify v0.16.0) 0adb851 (2020-12-15). Built on 2020-12-30. Build ID: KLdkAvgt
[2021-11-09T10:44:44Z INFO  librespot_core::session] Connecting to AP "gae2-accesspoint-a-v84t.ap.spotify.com:4070"
[2021-11-09T10:44:45Z INFO  librespot_core::session] Authenticated as "xxxxx" !
[2021-11-09T10:44:45Z INFO  librespot_playback::mixer::alsamixer] Setting up new mixer: card:default mixer:PCM index:0
thread 'main' panicked at 'Couldn't find simple mixer control for PCM,0', playback/src/mixer/alsamixer.rs:39:44

Trace here: https://paste.ubuntu.com/p/pNYKMD4gd8/

root@6bf06b9dadaf:/# aplay -L
null
    Discard all samples (playback) or generate zero samples (capture)
default
plugequal
equal
hw:CARD=Headphones,DEV=0
    bcm2835 Headphones, bcm2835 Headphones
    Direct hardware device without any conversions
plughw:CARD=Headphones,DEV=0
    bcm2835 Headphones, bcm2835 Headphones
    Hardware device with all software conversions
sysdefault:CARD=Headphones
    bcm2835 Headphones, bcm2835 Headphones
    Default Audio Device
dmix:CARD=Headphones,DEV=0
    bcm2835 Headphones, bcm2835 Headphones
    Direct sample mixing device

root@6bf06b9dadaf:/# aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Headphones [bcm2835 Headphones], device 0: bcm2835 Headphones [bcm2835 Headphones]
  Subdevices: 8/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
  Subdevice #7: subdevice #7