Set container_name field for mattermost and postgres containers

[HerManNav]

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.

[haivala]

Just create docker-compose.override.yml and set the names in there for your instance. They are not defined for a reason.

[HerManNav]

Ok. But then you should do this every time you update your mattermost instance version after the git pull.

In any case, if that is the intended way to go, I think it should be added to the documentation for newcomers to know at https://docs.mattermost.com/install/install-docker.html

[haivala]

git pull does not remove that file as it is not defined in the repo.