Bugfix: Reading environment from files didn't work for management commands

[stumpylog]

Proposed change

When reading files to populate environment variables, the exported variables aren't available when running a management command, as that's a different shell.

Refactors the reading of files to populate environment into a seperate script, which is then used in the docker startup and the management command.

Also expands the script to use anything ending in _FILE, meaning all environment can be set via a file if one desires.

Fixes #2248

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please explain)

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[shamoon]

Cool, love how it’ll be used in both places

[github-actions]

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