I updated your post to format the logs and the command. Please, follow this guide next time to make the message readable: How to format your forum posts
Which version of Docker are you using and on what OS? The output of docker info and docker version could help. Remove any details from the output you wouldn’t share.
About the shared error message: Is there anything at the end of the message afer “serve>”? If you add the --no-pager flag to journalctl it should return the whole output.
You should also go back to a point when ther were other kind of error messages that can explain why you see the DNS query error now. The --since parameter of journalctl expects a timestamp if you can scroll up enough to odler logs. It is also worth to check system logs of other services to see what happened before the error first occured so checking the logs without -u docker.service could be a good idea as well.
You mean it actually disconnected and when you checked the docker network inspect NETWORKNAME you didn’t see the container in the list of connected containers? I don’t think Docker would do that, but I could imagine an event that causes the network to disconnect. For example some cronjob or other event changes the network settings or an antivirus, firewall changes something.