How can the issue "host not found in upstream" be resolved in Docker Networking with nginx?

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There can be several reasons for the error "host not found in upstream" in Docker Networking with nginx. Some of the possible solutions are:

1. Ensure that the upstream server is running and accessible from the container network. Check if the IP address and port number specified in the upstream configuration are correct.

2. If the upstream server is running on a different host, ensure that the host is reachable from the container network. Check if the host's DNS name is resolving correctly.

3. If the upstream server is running in a different container, ensure that the containers are connected to the same network, and the upstream container is reachable from the nginx container.

4. If the upstream server is running in a swarm service, ensure that the service is published to the same network as the nginx container.

5. Check if the nginx.conf file has the correct upstream configuration. Ensure that the server name and port number match the ones specified in the upstream configuration.

6. Restart the nginx container to ensure that the latest configuration changes are applied.

In summary, the "host not found in upstream" error can be resolved by checking the accessibility of the upstream server, verifying the containers' network connection, and ensuring that the configuration files are correctly configured.