Update install.func

Check to see if the apt-cacher-ng proxy is available before attempting to use it. If it's not available, the system will directly communicate with the apt repositories.

misc/install.func

@@ -154,7 +154,16 @@ network_check() {
 update_os() {
   msg_info "Updating Container OS"
   if [[ "$CACHER" == "yes" ]]; then
-    echo "Acquire::http { Proxy \"http://${CACHER_IP}:3142\"; };" >/etc/apt/apt.conf.d/00aptproxy
+    echo "Acquire::http::Proxy-Auto-Detect \"/usr/local/bin/apt-proxy-detect.sh\";" >/etc/apt/apt.conf.d/00aptproxy
+    cat <<EOF >/usr/local/bin/apt-proxy-detect.sh
+#!/bin/bash
+if nc -w1 -z "${CACHER_IP}" 3142; then
+  echo -n "http://${CACHER_IP}:3142"
+else
+  echo -n "DIRECT"
+fi
+EOF
+  chmod +x /usr/local/bin/apt-proxy-detect.sh
   fi
   $STD apt-get update
   $STD apt-get -o Dpkg::Options::="--force-confold" -y dist-upgrade