use CentOS' CA certificate bundle

when mounting the configuration from contrib, as suggested in the
README, the auth proxy will not start unless a CA certificate bundle
is present.
This change circumvents this problem and makes a possible solution to
this problem more visible.

contrib/authproxy.cfg

@@ -6,6 +6,8 @@
 [main]
 # Provide meaningful output for `docker logs <cid>`.
 log_stdout=true
+# use the CA certificates file supplied by the OS
+http_ca_certs_file=/etc/ssl/certs/ca-bundle.crt
 
 # Go through a corporate proxy to reach duo.
 http_proxy_host=proxy.example.com