Explain https forward environment in documentation

[trucheromayor]

The issue detected in this mapproxy ticket was solved by adding an "http-request add-header X-Forwarded-Proto https if { ssl_fc }" line to happroxy. Would be great if this scenario is explained somewhere in the documentation.

Here is a proposal:

When working behind a fronted like haproxy serving HTTPS scheme and redirecting requests to a mapproxy backend you will need to inform your backend about your schema with a X-Forwarded-Proto HTTPS header. Example of configuration in haproxy:

http-request add-header X-Forwarded-Proto https if { ssl_fc }

Thank you in advance

Jon