Regarding the Docker version (which is Ubuntu): I had to build the container with an updated ca-certificates package and everything worked perfectly from that point forward. I guess the pre-built containers are using an old package? Just to be sure, I also included the ldap-utils package so I could manually run tests like ldapsearch from within the container.
I gave more details in this post since I was also just facing this issue.
I hope this helps, good luck!