add squid configuration

config/dune-proxy/squid.conf

+acl CONNECT method CONNECT
+
+acl good_domains dstdomain gitlab.dune-project.org
+
+acl good_ports port 443
+
+http_access allow CONNECT good_domains good_ports
+http_access deny all
+
+http_port 3128
+visible_hostname dune-proxy
+
+pinger_enable off
+
+access_log none
+cache_log /dev/stdout
+
+# use upstream proxy
+cache_peer 172.22.1.3 parent 3128 0 no-query default
+never_direct allow all