Cookiecutter-django: Update Docker backup docs

[fluid217]

Description

In docs/docker-postgres-backups.rst it's suggested to use:

docker-compose -f local.yml exec postgres backup

I'd suggest updating that to:

 docker-compose -f local.yml exec -T postgres backup

Rationale

Running the command as documented from a Cron job fails with:
`` The input device is not a TTY. ```` This can be fixed by adding the above-T` switch.

Use case(s) / visualization(s)

Works fine as documented if backing up by hand. Cron jobs however have potential to fail.

[browniebroke]

Sounds good. I wouldn't include this option in all commands, but I think it would be great to add a paragraph at the end of the page on how to run backups in a crontab.

We could link to some external documentation for those unfamiliar with cron and maybe suggest an entry for daily backups.