Edit the Docker using the advanced view (toggle in the top-right). Now you can change the Extra Parameters to add the ones missing for Cloud support. Mine looks like this (just replace the redacted bits with the ones shown in your Netdata Cloud account):
--restart unless-stopped --cap-add SYS_PTRACE --security-opt apparmor=unconfined -e NETDATA_CLAIM_TOKEN=###redacted_super_secret_token_value### -e NETDATA_CLAIM_URL=https://app.netdata.cloud -e NETDATA_CLAIM_ROOMS=###redacted_super_secret_room_value### --log-opt max-size=200m --log-opt max-file=1