dockerfile: specify `ENTRYPOINT` as `entrypoint.sh`

Dockerfile

@@ -23,4 +23,4 @@ RUN chmod 755 /sbin/entrypoint.sh
 
 EXPOSE 53/udp 10000/tcp
 VOLUME ["${DATA_DIR}"]
-CMD ["/sbin/entrypoint.sh"]
+ENTRYPOINT ["/sbin/entrypoint.sh"]

entrypoint.sh

@@ -48,4 +48,9 @@ fi
 echo "Starting named..."
 mkdir -m 0775 -p /var/run/named
 chown root:${BIND_USER} /var/run/named
-exec /usr/sbin/named -u ${BIND_USER} -g
+
+if [ -z "$@" ]; then
+  exec /usr/sbin/named -u ${BIND_USER} -g
+else
+  exec "$@"
+fi