Fix a bug that config.json is not created

Do not copy config.json into /matteronst/config directly because:

> Changes to a data volume will not be included when you update an image.
>
> -- https://docs.docker.com/engine/userguide/dockervolumes/

Instead copy config.template.json into the root and generate
/mattermost/config/config.json from the file.

app/Dockerfile

@@ -14,12 +14,10 @@ exec bin/platform\n'\
 RUN wget https://github.com/mattermost/platform/releases/download/v1.2.1/mattermost.tar.gz \
 	&& tar -xvzf mattermost.tar.gz && rm mattermost.tar.gz
 
-ADD config/config.json /mattermost/config/
-ADD docker-entry.sh /
+COPY config.template.json /
+COPY docker-entry.sh /
 
 RUN chmod +x /docker-entry.sh
 ENTRYPOINT /docker-entry.sh
 
-VOLUME ["/mattermost/config"]
-
 EXPOSE 80