Add a shim to znc

This serves to do two things:

  1) chown /znc to znc:znc, which is really handy if the znc
     user inside the docker container has a new uid, because the
     files are stored on the host filesystem.
  2) to drop privs to the znc user, to reduce the attack surface.

znc/znc-shim

@@ -0,0 +1,11 @@
+#!/bin/bash
+set -e
+
+ZNC_DIR="/znc"
+ZNC_OWNER=$(stat -c "%U" ${ZNC_DIR})
+
+if [ "${ZNC_OWNER}" != "znc" ]; then
+    chown -R znc:znc ${ZNC_DIR}
+fi
+
+exec sudo -u znc "$@"