You can tweak nginx configuration inside PMM as it shown in this stackoverflow question Pass cert password to Nginx with https site during restart - Stack Overflow, but it will not survive upgrade, so you will have to tweak it after each upgrade. Another option to use certificate with no passphrase.
I was able to convert the key to passwordless and create a dhparam.pem but it’s still not right.
The command:
docker exec -it pmm-server chown root.root /srv/nginx/*
returned
chown: cannot access ‘/srv/nginx/*’: No such file or directory
so I used the command for each file explicitly.