Add an Alpine Linux-based Docker image

It gets all it's dependencies (Mopidy, Mopidy-Spotify) from the Alpine
Linux repositories.

For reference the image sizes:
- Debian, 1.49GB
- Alpine, 832MB

docker/entrypoint.sh

@@ -1,14 +1,14 @@
-#!/bin/bash
+#!/bin/sh
 
 if [ -z "$PULSE_COOKIE_DATA" ]
 then
-    echo -ne $(echo $PULSE_COOKIE_DATA | sed -e 's/../\\x&/g') >$HOME/pulse.cookie
-    export PULSE_COOKIE=$HOME/pulse.cookie
+    printf '%s' "$(echo "$PULSE_COOKIE_DATA" | sed -e 's/../\\x&/g')" >"$HOME"/pulse.cookie
+    export PULSE_COOKIE="$HOME"/pulse.cookie
 fi
 
 if [ ${PIP_PACKAGES:+x} ]; then
     echo "-- INSTALLING PIP PACKAGES $PIP_PACKAGES --"
-    python3 -m pip install --no-cache $PIP_PACKAGES
+    python3 -m pip install --no-cache "$PIP_PACKAGES"
 fi
 
 exec "$@"