Ensure PostgreSQL is stopped gracefully in build-verify-image

What does this merge request do and why?

In the build-verify-image CI job, we do a lot of database work, such as seeding the database and truncating tables, and then abruptly kill all processes. This leads to many outstanding write-ahead log (WAL) records, causing a long recovery process to occur after the database is started up next.

This was discovered in !3308 (comment 1507154071).

Relates to #1910 (closed)

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.