title: Docker use host port without mapping port
body: <p>I want to send emails from 2 separate docker containers. Both of these containers need port 25 of the host system but I can only map the port to one of those containers. When I try with the second one it throws this error: <code>Bind for 0.0.0.0:25 failed: port is already allocated</code>. How do I use the port of the host system without actually mapping it to one of the containers? The containers need to run on a certain network and not on the host network. I'm using Nodemailer in the docker containers btw.</p>