Booksonic Container // behind reverse proxy

Dear all
I’m running the booksonic container on a Synology machine. Now I try to set up the Synology Reverse Proxy to access the booksonic application not by http but https.

That’s my problem: I do not know the setting to reach the login page.

Using “http://myDomain.tld:4040” worked out fine. I get to “login/view?”
Using “https://booksonic.mydomain.tld” doesn’t work. If I use the e-variable " <<CONTEXT_PATH>> with booksonic or booksonic/login/view? I get a message 404 “can’t find page” from Synology.

My settings for the Synology Reverse Proxy I tried to follow as close as possible to my calibre setting (as this seems to work)

Within the Synology setup it is possible to define a user-defined header: for calibre I used
www.authenticate / Basic real=“RealmName”
Upgrade / $http_upgrade
Connection / $connection_upgrade

I would be happy if you could suggest any help with the necessary settings for booksonic.

with regards

ps - the information for basic authentication (www.authenticate / Basic real=“RealmName”) I found on wikipedia: