At present, Portainer is built on the one server multiple agents model where you have one environment that is your "management" interface and runs Portainer Server, and the others use the Agent to interface with the Server. You can only log into the environment running the Portainer Server container, not the Agents. We don't currently support multi-tenancy of the Portainer Server.
In production setups we generally recommend a separate environment purely for management, where workloads don't run, that you run the Portainer Server on, and all workload environments use Agents. This way if one of your workload environments goes down, you can still manage the others in the meantime.