Upgrade script fails if mattermost-docker is used with .env

[lu40]

While migrating to the new PostgreSQL and Mattermost versions using the upgrade-from-mattermost-docker.sh script, we encountered a problem with the parsed environment variables. The docker command to upgrade the db at L179-L186 of the script fails with the following message:

initdb: option requires an argument -- 'U'

This is only an assumption as we haven't been reproducing this error, but we guess that the problem lies in having used variables from .env with mattermost-docker like this:

environment:
  - POSTGRES_USER=${POSTGRES_USER}
  - POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
  - POSTGRES_DB=${POSTGRES_DB}

So when exporting the variables in the shell the docker command worked as expected.

Thanks a lot for the great work you put into restructuring the docker setup for mattermost.

[mrckndt]

@lu40 thank you for reporting. Because there are efforts in improving the upgrade i'm closing this in favor of #25.