libldap-common is missing in beanbag/reviewboard:6.0 and above. This has a nasty consequence: Verification of LDAPS servers with non-public certificates is not possible anymore! Instead, RB will just report the very unspecific error:

Error authenticating with LDAP: {'result': -1, 'desc': "Can't contact LDAP server", 'ctrls': [], 'info': '(unknown error code)'}
Solution proposal:

Add apt-get install libldap-common to Dockerfile, so the package gets installed explicitly and LDAP client uses system-wide certificate store again.

Built a docker image derived from beanbag/reviewboard:6.0, installed the missing package libldap-common and add appropriate non-public certificate to system-wide certificate store
Run container based on derived image
RB UI: Configure LDAP auth to a server that uses a non-public certificate
RB UI: Try to login with LDAP-based user account

-> No error message seen anymore and login succeeded.