Consider adding entrypoint to check for database connection prior to running

[spwoodcock]

Is your feature request related to a problem? Please describe.

• Typically this service would run with a depends_on and and healthcheck for a database, waiting until the db is healthy before running.
• When extending the official ODK Central stack, the database does not have a healthcheck.
• We encounter an error connecting while it initialises, before showing the successful connection on retry:

Describe the solution you'd like

• Assess possibility of lightweight healthcheck for database, prior to running this service.

Describe alternatives you've considered

• The postgres14 service in the official stack could simply be extended to add a healthcheck.

Additional context

• This might require adding a shell, which I'm not that keen on.
• Currently the image is built on scratch, meaning no sh or bash present, for optimal security.