When running owncloud/server:10.15 in the docker compose container as described in the docs here, a warning is shown under Admin / General / Security & setup warnings.
The "Strict-Transport-Security" HTTP header is not configured to at least "15552000" seconds. For enhanced security we recommend enabling HSTS as described in our security tips. Please double check the installation guides, and check for any errors or warnings in the log.
I wanted to make a bug report about this topic, but the github page mentioned to post it here first. In my opinion, this is a misconfiguration in the owncloud docker image.