Use wait-for-it in favor of the custom python script when waiting for postgres

[foarsitter]

Proposing the usage of wait-for-it in favor of our own python script as raised in #4539

wait-for-it can also be used in the healthcheck command but that is out of scope for now.

[luzfcb]

Does wait-for-it ensure that host + port is reachable and the host + port + user + password + database set is 100% functional and can execute queries?

As far as I know, it is one thing is the PostgreSQL process to be active on a certain port, and another thing is PostgreSQL to be active and able to answer queries for a specific set of user + password + database.

Specifically for health checks, there is a case where the PostgreSQL process is active on a port, but is not yet ready to receive queries. This may take a few milliseconds or seconds. That said, I don't remember where I read this. Maybe I'm wrong.

[foarsitter]

@luzfcb interesting remark! wait-for-it checks if a port accepts TCP traffic, nothing more.

For postgres healthchecks I would relay on pg_isready but the django container doesn't have postgresql-client installed so that isn't possible.

I'm testing wait-for-it localhost:5000 as a healthcheck for the django container at the moment.

Considering that the boot time of django will take a few seconds I don't think we will get in trouble here testing without credentials for postgres.