Got an answer from Andras, the Greek dictionary should be installed in the container, as I can verify too:
docker run --rm quay.io/egroupware/collabora-key dpkg -l | grep dict-el
ii collaboraoffice6.2-dict-el 18.104.22.168-10 amd64 El dictionary for Collabora Office 6.2 -10
Andras also said, while all available LibreOffice dictionaries are installed in the container, only a couple are preloaded and Greek is not one of them:
After adding Greek to the preloaded dictionaries in my docker-compose.yml:
- dictionaries=de_DE en_GB en_US es_ES fr_FR it nl pt_BR pt_PT ru el
AND restarting the container, I could spell-check a Greek word I googled: φιλοξενία
For the environment variable to work (patching it into loolwsd.xml), the /etc/loolwsd mount must NOT be readonly!
Can you verify that eg. by stoping and deleting (rm -f) the collabora container and starting it again (up -d).