dev-docs: add on-prem terraform to vpn setup (#2619)

* vpn: add fake-on-prem infra

* dev-docs: move vpn helm

dev-docs/howto/vpn/helm/files/strongswan/entrypoint.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+# The charon binary is not included in the PATH generated by nixery.dev, find it manually.
+charon="$(dirname "$(readlink -f "$(command -v charon-systemd)")")/../libexec/ipsec/charon"
+
+"${charon}" &
+
+while ! swanctl --stats > /dev/null 2> /dev/null; do
+  sleep 1
+done
+swanctl --load-all
+
+wait