I was using a dedicated IP by way of "custom: br0" and had the same issue, my routing looked fine from the unRaid side. Most other containers don't have any issues, but I have ran across a few that insist on running on bridge... So change your network type to bridge and see if it fixes if for you...
Edit: Got it to work with custom IP by adding the "--dns 1.1.1.1" in the extra parms in the docker config...