Dockerfile - lowercase ARG names, for consistency

Dockerfile

@@ -4,8 +4,8 @@ FROM alpine:$alpine_version
 # retrive alpine_version from before FROM
 ARG alpine_version
 
-ARG PUID=1000
-ARG PGID=1000
+ARG puid=1000
+ARG pgid=1000
 # This is an archive and isn't updated any more
 ARG spotify_version=12.1.51-Linux-x86_64
 
@@ -61,8 +61,8 @@ RUN \
       Mopidy-Spotify-Web \
       Mopidy-SoundCloud \
   && echo "* Creating Mopidy User" \
-    && addgroup -g ${PGID} mopidy \
-    && adduser -h /mopidy -s /bin/sh -D -G mopidy -u $PUID mopidy \
+    && addgroup -g ${pgid} mopidy \
+    && adduser -h /mopidy -s /bin/sh -D -G mopidy -u ${puid} mopidy \
     && 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/* \