How to change the default port used by postfix

[zac]

I am using Google Cloud Engine and the port 25, the default port used by postfix, is blocked.
I tried changing the port in master.cf but that is unfortunately not working.
I also tried using Gmail as a postfix relay but that also didn’t work.
I used docker inspect to view the postfix container and found the ports are defined there. So I assume an changes I make in .cf file regarding ports will be overridden by the docker configuration.
Is there any way I can change them?
Any help would be much appreciated.

[zac]

update: I’ve managed to change the exposed port to 587 but still unable to send emails
Steps I followed:

1. Find the docker ID, name and port of the relevant container
   docker ps --format "table {{.Names}}\t{{.ID}}\t{{.Ports}}"
2. Note the container ID(ex: 7c79d07c0c6b) and name (ex: examplecom_postfix_1) of the postfix container corresponding to your site
3. Stop the container
   docker stop examplecom_postfix_1
4. Find the full container ID by listing files
   ls /var/lib/docker/containers/
5. You’ll be presented with a list of long directory names. You have to select the one that starts with the container ID you noted down earlier and edit config.v2.json
   nano /var/lib/docker/containers/7c79d07c0c6b......./config.v2.json
6. Change the exposed port and save the file
7. Restart docker
   systemctl restart docker
8. Start the docker container
   docker start examplecom_postfix_1
9. Verify the port has indeed been changed
   docker ps --format "table {{.Names}}\t{{.ID}}\t{{.Ports}}"