To specify DNS in the installation of the OnlyOffice container use the following form:
docker run -dit -p 88:80 --dns=80.80.80.80 --restart always --name oods onlyoffice/documentserver
substituting --dns as required. Apparently OnlyOffice uses Google 8.8.8.8 by default.
More on DNS and OnlyOffice in Seafile can be found here and here.
See Container Networking (DNS services) for more on configuring DNS with Docker containers.
I hope this helps.