Test plan for automatic Postgres upgrades in the server image

Created by: tsenart

We'd like to test both the documentation and the automated upgrades.

Go to https://github.com/sourcegraph/sourcegraph/blob/master/doc/admin/migration/3_0.md#bundled-postgres-upgrade and test automated and manual upgrades to 3.0.1-rc.3 coming from 2.13.6, 3.0.0 and 3.0.0-beta5 3.0.0-beta.4.

You can create a fresh install of each of these versions by running the correspondent Docker container. Ensure you clean-up your data directory between each test (e.g. rm -rf ~/.sourcegraph)

Here's a check-list for you to copy into a comment in this issue and tick as you go:

- [ ] Test manual upgrade from `2.13.6` to `3.0.1-rc.3`.
- [ ] Test manual upgrade from `3.0.0` to `3.0.1-rc.3`.
- [ ] Test automatic upgrade from `2.13.6` to `3.0.1-rc.3`.
- [ ] Test automatic upgrade from `3.0.0-beta.4` to `3.0.1-rc.3`.
- [ ] Test automatic upgrade from `3.0.0` to `3.0.1-rc.3`.

Part of #1404 (closed)