In order to redirect all HTTP traffic of a HAProxy Rancher load-balancer to HTTPS, you must upgrade your load-balancer service for :

  1. adding one HTTP rule per website ;
  2. adding one HTTPS rule per website ;
  3. specifying websites SSL certificate.
  4. writing custom HAProxy rule for HTTP to HTTPS redirection.

This latest point is focused by this article. For that, on HAProxy load-balancer upgrade view, just click on Custom haproxy.cfg tab :

Rancher HAProxy Custom conf

... then, add this rule :

frontend 80
  http-request redirect scheme https code 301 if !{ ssl_fc }

Simply validate with Edit button.

Every HTTP website access on this load-balancer is now redirected to HTTPS. ;)