Documentation changes

[dilbernd]

Hi, first of all many thanks for the stellar work; the images scratch a huge itch. Upgrading docker envs with Postgres 13 which has less than a year now were unexpectedly breezy.

I’d just advise some changes to the readme on docker hub.

1. The upgrade potentially creates hints/script files e.g. for extension upgrades which get lost with --rm.

I’d remove the --rm from the recommended run and note that the corresponding files are available in the "upper dir" of the container after run, or even advise also mounting the upgrader work dir in.

2. For sufficient paranoia, --network none might instead be advisable since the upgrade does not need connectivity and it removes exfil concerns for closed envs.

3. pg_hba for docker envs, which will likely be a good part of the user base, needs some adaptation after upgrade.

4. potentially also the postgresql.conf? How do you handle that for your upgrades?

If you’d like to save time on these docs changes, I’d be happy to make the changes if you’re willing to accept pull reqs.