initramfs hook for tailscale

This is intended to be used with an ephemeral auth key with an ACL tag,
and ACL rules that restrict the ephemeral node to inbound-only traffic.
It does not share instance state with tailscale running in Linux.

Reference:
- https://tailscale.com/kb/1111/ephemeral-nodes/
- https://tailscale.com/kb/1068/acl-tags/#generate-an-auth-key-with-an-acl-tag
- https://tailscale.com/kb/1068/acl-tags/#using-tags-in-acls-for-access-control

debian/rules

@@ -0,0 +1,14 @@
+#!/usr/bin/make -f
+
+#export DH_VERBOSE = 1
+
+%:
+	dh $@
+
+execute_after_dh_fixperms:
+	chmod 600 debian/tailscale-initramfs/etc/tailscale/initramfs/config
+
+override_dh_builddeb:
+	# Workaround for building on Ubuntu and installing on Debian (Ubuntu uses
+	# zstd). https://bugs.debian.org/892664
+	dh_builddeb -- -Zxz