postgres-upgrade: halt if any stage of upgrade fails

[alxndrsn]

Noted while working on #882: files/postgres/upgrade-postgres.sh is not actually run with /bin/bash -eu as its shebang declares, but with /bin/sh.

This PR

1. fixes the shell used for upgrade-postgres.sh
2. introduces a test which would have detected this issue, and should prevent similar issues in future
3. fixes use of deprecated dockerfile syntax detected by the new test
4. pulls shellcheck invocation into a script file to allow easier use locally, and possible future extension

Docker build checks

Currently in beta, "docker build checks" are a best-practice linter for dockerfiles.

See: https://docs.docker.com/build/checks/#check-a-build-without-building

Dockerfile fixes

• postgres-upgrade.dockerfile: https://docs.docker.com/reference/build-checks/json-args-recommended/
• postgres14.dockerfile: https://docs.docker.com/go/dockerfile/rule/legacy-key-value-format/

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Related: #342

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What has been done to verify that this works as intended?

CI

Why is this the best possible solution? Were any other approaches considered?

Docker build checks are built-in to docker. However, they're in beta, so there is a chance they could be changed or dropped in future. This PR could be restricted to simply changing postgres-upgrade.dockerfile.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Should not affect users.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced