app: Make it works without volume mounting

This fixes https://github.com/mattermost/mattermost-docker/pull/32#issuecomment-226408765.

app/Dockerfile

@@ -5,9 +5,10 @@ RUN mkdir -p /mattermost/data
 
 RUN curl https://releases.mattermost.com/3.0.2/mattermost-team-3.0.2-linux-amd64.tar.gz | tar -xvz
 
+RUN rm /mattermost/config/config.json
 COPY config.template.json /
-COPY docker-entry.sh /
 
+COPY docker-entry.sh /
 RUN chmod +x /docker-entry.sh
 ENTRYPOINT ["/docker-entry.sh"]