fix: remove config volume mount from db container

[sweatybridge]

What kind of change does this PR introduce?

Bug fix #4571

What is the new behavior?

Since we started injecting pgsodium root key via container entrypoint, there's no need to volume mount the /etc/postgresql-custom directory.

This resolves the problem where new directories are added to this path from upstream postgres image, hence breaking local upgrades.

Additional context

relates to #1212
supersedes #4570

[coveralls]

Pull Request Test Coverage Report for Build 19919439447

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.03%) to 56.155%

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Files with Coverage Reduction	New Missed Lines	%
internal/storage/rm/rm.go	2	80.61%
internal/gen/keys/keys.go	5	12.9%

Totals
Change from base Build 19919068534:	-0.03%
Covered Lines:	6815
Relevant Lines:	12136

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💛 - Coveralls