Upgrading from mattermost-docker

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Hello @jasonblais , @mrckndt
As mentioned in https://github.com/mattermost/mattermost-docker/pull/529 I did my tests to migrate from MM 5.31 to 5.32.1 with the migration script.

I have few remarks about the script :

1. When he try to get the POSTGRES_OLD_VERSION from the docker container he gets : 9.4.21 instead of 9.4 so when you try to find the docker image it results in :
```
docker: Error response from daemon: manifest for tianon/postgres-upgrade:9.4.21-to-13 not found: manifest unknown: manifest unknown.
```
So I suggest that the version must be fixed in the script or export before running it.

2. Same for the MM_VERSION when we do the docker-compose build. When the version is commented like that ` # - MM_VERSION=5.31` in the docker-compose.yml and in the Dockerfile you have : ARG MM_VERSION=5.31.0
the `sed` command will write `5.32.1.0` instead of `5.32.1`

so what do you think about reading the value directly from the Dockerfile instead of the docker-compose.yml file ?

Then the last step, when the migration is done, how to swith from the old directories/files structures to the new one with the new repo ? Is it possible to change the path for the volume directly in the .env file and that's all ?

Thank you very much.

Best regards,

Start with the migration script, docker-compose.yml, Dockerfile, and .env entries mentioned in the report. Reproduce the PostgreSQL image lookup and MM_VERSION substitution during the 5.31-to-5.32.1 migration, then investigate how the new repository's volume paths should be configured. Done means the migration handles the reported versions and documents or verifies the final volume-path transition.

Written by the indexing model from the issue text.