r/lego-nginx: Configure LEGO for nginx

The *lego-nginx* role automates obtaining certificates for *nginx* via
ACME using `lego`.  It generates a shell script with the appropriate
arguments for `lego run`, runs it once to obtain a certificate
initially, then schedules it to run periodically via a systemd timer
unit. Using `lego`'s "hook" capability, the script signals the `nginx`
server process to reload.  This uses `doas` for now, but could be
adapted easily to use `sudo`, if the need ever arises.

roles/lego-nginx/files/lego.nginx.conf

@@ -0,0 +1,3 @@
+location ^~ /.well-known/acme-challenge {
+    root /var/www/lego;
+}