Problems rewriting urls with containers and reverse proxy

[PaulOchons]

Hello,
I have installed formtools in containers (one for the application and a second one for the database). The containers are behind a reverse proxy.
The installation works fine, as does the creation of the admin account, but I have a problem when I try to log in.
The url of my application is https://mywebite.com/formtools/ but when I click on the login button I'm redirected to https://mywebsite.com/index.php and so it doesn't work.

In the config.php file I have:
$g_root_url = "https://mywebite.com/formtools";
$g_root_dir = "/var/www/html";

If I consult the source code of the login page I see this:
form name="login" action="/index.php" method="post"
Whereas it should be:
form name="login" action="/formtools/index.php" method="post"

Can you see where this problem might be coming from?
Is it more a problem with the proxy than with the formtools application?

Thanks !