Hi @aalten, since Mattermost Docker installation is using nginx as a reverse proxy, you don't need to setup MM to use the cert and key. The SSL is handled by nginx and off-loaded when nginx forward the request to MM.
Which version of MM docker are you using? If you cloned/checked out the repo before this (https://github.com/mattermost/mattermost-docker/commit/f7c78e675a20d01e18cf28c41d8a5a383c4d037b) commit, you need to set MATTERMOST_ENABLE_SSL=true in docker-compose.yml. Or you can just checkout the latest version of MM docker repo. You will need to rebuild the images. (docker-compose up -d --build)