Update the PostgreSQL and Redis docker images

[justinclift]

Updates the version of PostgreSQL we use to version 15

[justinclift]

Thanks for reviewing this @tinder-tder. It's probably better for us to hold off on merging it until we've got all the data source libraries being correctly built and included in our preview docker image though. Just so we don't add any unnecessary extra complication (eg to dql).

[justinclift]

Just updated this PR to use Redis 7 instead. I'm half expecting it to fail, so lets see what happens. 😄

[justinclift]

Well, it worked. Going to merge it, and lets see how it goes... 😄

[justinclift]

When we create a proper release that includes this PostgreSQL upgrade, we'll need to make sure there's clear communication about it.

For users with an existing Redash installation, they'll need to run an extra step to upgrade their PostgreSQL on-disk data format.

https://www.postgresql.org/docs/current/pgupgrade.html

Ideally, we'd figure out some good way to automate that so people don't have to do anything manually.