docker exec is not working in cron

Your docker exec command says it needs "pseudo terminal and runs in interactive mode" (-it flags) while cron doesn't attach to any TTYs.

Try changing your docker exec command to this and see if that works?

docker exec mongodb mongodump -d meteor -o /dump/

for what it's worth I had this exact same problem. Fixing your PATH, changing permissions, and making sure you are running as the appropriate docker user are all good things, but that's not enough. It's going to continue failing because you are using "docker exec -it", which tells docker to use an interactive shell. Change it to "docker exec -t" and it'll work fine. There will be no log output anywhere telling you this, however. Enjoy!