Dockerfile, mopidy.conf - add mopidy user to audio group

So that it can use the ALSA sound device directly

(This also goes for icecast, which uses the same user `mopidy`)

Dockerfile

@@ -6,6 +6,8 @@ ARG alpine_version
 
 ARG puid=1000
 ARG pgid=1000
+ARG audio_gid=29
+
 # This is an archive and isn't updated any more
 ARG spotify_version=12.1.51-Linux-x86_64
 
@@ -62,7 +64,10 @@ RUN \
       Mopidy-SoundCloud \
   && echo "* Creating Mopidy User" \
     && addgroup -g ${pgid} mopidy \
-    && adduser -h /mopidy -s /bin/sh -D -G mopidy -u ${puid} mopidy \
+    && adduser -h /mopidy -s /bin/sh -D -G mopidy -u $puid mopidy \
+    && delgroup audio \
+    && addgroup -g ${audio_gid} audio \
+    && addgroup mopidy audio \
     && echo "mopidy ALL=NOPASSWD: /usr/local/lib/pyenv/versions/*/lib/python*/site-packages/mopidy_iris/system.sh" >> /etc/sudoers \
   && echo "* Cleaning up" \
     && rm -rf /var/cache/apk/* \