* add instructions and container to create a key and certificate pair
for the built-in IDM LDAP as we need it to listen on LDAPS for
Stalwart, which has been disabled by default with
https://github.com/opencloud-eu/opencloud/pull/2880
* add environment variables to point to the certificates
* update services/groupware/DEVELOPERS.md with up-to-date instructions,
using Stalwart 0.16
75f062f53d