When you create a new container, you can specify one or more --network flags.
Consider the following example , this connects a Nginx container to the my-net network. It also publishes port 80 in the container to port 8080 on the Docker host, so external clients can access that port. Any other container connected to the my-net network has access to all ports on the my-nginx container, and vice versa.
$ docker create --name my-nginx \
--network my-net \
--publish 8080:80 \
To connect a running/existing container to an existing user-defined bridge, use the docker network connect command. The following command connects an already-running my-nginx container to an already-existing my-net network:
$ docker network connect my-net my-nginx