Set container_name field for mattermost and postgres containers

This is a minor, but:

I think we should consider to set the container_name field for both postgres and mattermost images (in docker-compose.yml), as it is already done in docker-compose.nginx.yml, instead of letting them to be built by the docker-compose command.

I realized about this need since I had 2 different machines for 2 Mattermost servers. The first one had docker v20.10.14. The second one had a minor update v20.10.16, since I set it up a bit later. With the first one I got the following names:

• mattermost-mattermost-1
• mattermost-postgres-1

while with the second one I got the same names but concatenated with underscores instead of dashes.

Then I wanted to used the very same mmctl commands for both machines... but needed to change the container name every now and then.

Who knows if docker-compose changes again in the future to follow another naming convention, not only regarding the dashes or underscores. I propose to set those names in the docker-compose.yml to fixed names from now on.

Open docker-compose.yml and compare its mattermost and postgres services with the corresponding entries in docker-compose.nginx.yml. Done means both services have fixed container_name values so the same mmctl commands can address them consistently across Docker Compose versions.