Great, thank you eungjun.yi, that's done the trick...
All 3 certificates are now in
/home/myuser/mattermost-docker/volumes/web/cert/fullchain.pem
/home/myuser/mattermost-docker/volumes/web/cert/cert.pem
/home/myuser/mattermost-docker/volumes/web/cert/key-no-password.pem
And key-no-password.pem is just a renamed copy of privkey.pem from the Let's Encrypt certificates...