fix(db): set working directory for SQL relative imports

[7ttp]

Summary

Fixes SQL relative imports in test files by setting the container working directory.

Problem

SQL files using \i fail with "No such file or directory" because the container has no working directory set.

Solution

Set WorkingDir to the parent directory of the test file, allowing psql to resolve relative imports correctly.

Related

• Extends fix(db): support arbitrary test directories #4590
• Closes supabase db test not able to import extra relative file #4734

[7ttp]

@avallete @sweatybridge