Fix permissions issue with data dir

If the mounted data directory on the host is owned by root, the znc user
within the container would fail to write any settings to disk.

Hence this changes ownership of the data directory to the znc user (Uid:
1000) and znc group (Gid: 1000) from within the container.

Dockerfile

@@ -13,7 +13,6 @@ ADD         start-znc /usr/local/bin/
 ADD         znc.conf.default /src/
 RUN         chmod 644 /src/znc.conf.default
 
-USER        znc
 EXPOSE      6667
 ENTRYPOINT  ["/usr/local/bin/start-znc"]
 CMD         [""]