Add PORT and CONTEXT_PATH env variables

Dockerfile

@@ -2,6 +2,8 @@ FROM java:8
 MAINTAINER Cyrille Nofficial<cynoffic@cyrilix.fr>
 
 ENV SUBSONIC_VERSION 5.2.1
+ENV PORT 8080
+ENV CONTEXT_PATH /
 
 LABEL version="$SUBSONIC_VERSION"
 LABEL description="Subsonic media streamer"
@@ -13,18 +15,16 @@ RUN     mkdir -p /opt/subsonic  /opt/data /opt/music/ &&\
 VOLUME /opt/data
 VOLUME /opt/music/
 
-EXPOSE 8080
+EXPOSE $PORT
 WORKDIR /opt/subsonic
 
 CMD java -Xmx100m \
             -Dsubsonic.home=/opt/data \
-            -Dsubsonic.port=8080 \
-            -Dsubsonic.contextPath=/ \
+            -Dsubsonic.port=$PORT \
+            -Dsubsonic.contextPath=$CONTEXT_PATH \
             -Dsubsonic.defaultMusicFolder=/opt/music/ \
             -Dsubsonic.defaultPodcastFolder=/opt/music/ \
             -Dsubsonic.defaultPlaylistFolder=/opt/music/ \
             -Djava.awt.headless=true \
             -verbose:gc \
             -jar /opt/subsonic/subsonic-booter-jar-with-dependencies.jar
-
-