Some minor changes to hopefully make the sandbox Docker compose setup more stable.

• Lock the Postgres version to 15.1 to prevent breakage from new Postgres versions we haven't tested yet.
• Switch the Flyway image from centos to almalinux and lock the version. Centos images are no longer officially supported, and Almalinux seems to be the closest replacement.
  • This allows us to remove some yum repo customizations that were needed on Cent.
• Upgrade Flyway to a more recent version.
• Update the Flyway migration script to improve debugging:
  • Use set -e to fail the script when migrations (or any other step) fails. This should make failures more obvious; without it, the container finishes with exit code zero even when it has failed.
  • Print a message when starting migrations to make the log output more clear.
• Add a new script get-latest-release-tag.sh. This will be used by the quickstart guides to let users fetch the latest release version number. Currently this version number is hardcoded in the quickstart guides and needs to be updated every time there's a new release, which is easily forgotten.