We recently ran to a situation that we have a running container (MySQL) but we liked to access it from internet and this container didn't have any port mapping between host and container.
We spawn up the container with docker compose
One solution for this situation is to stop the container, remove it and execute docker run with -p <hostPort>:<containerPort> but we didn't want to create new container.
Second solution is using iptables to do the port mapping.
In the following example, you see how we achieved routing calls to the host:3306 to the mysqlContainer:3306.
You should run this on the host machine.
sudo iptables -t nat -A DOCKER -p tcp --dport 3306 -j DNAT --to-destination MysqlContainerIP:3306
sudo iptables -t nat -A POSTROUTING -j MASQUERADE -p tcp --source MysqlContainerIP --destination MysqlContainerIP --dport 3306
sudo iptables -A DOCKER -j ACCEPT -p tcp --destination MysqlContainerIP --dport 3306