I noticed, that when I specify port_bindings, the wait_for isn't being set properly.

Below you can find a step by step expanation why it's happening:

1. PostgresContainer constructor calls parent constructor https://github.com/testcontainers/testcontainers-ruby/blob/main/postgres/lib/testcontainers/postgres.rb#L34
2. DockerContainer constructor calls add_wait_for https://github.com/testcontainers/testcontainers-ruby/blob/main/core/lib/testcontainers/docker_container.rb#L59
3. The method parameter is nil and @exposed_ports field is present, so it sets a @wait_for that tries to connect to the bound port https://github.com/testcontainers/testcontainers-ruby/blob/main/core/lib/testcontainers/docker_container.rb#L270
4. Going back to PostgresContainer's constructor - it does a conditional assignment to @wait_for, but it has been already set to a non-nil value, even though no wait_for option was specified https://github.com/testcontainers/testcontainers-ruby/blob/main/postgres/lib/testcontainers/postgres.rb#L39

This can be worked around by specifying the wait_for option explicitly

Testcontainers::PostgresContainer.new(
  'postgres:16',
  username: test_db_config['username'],
  password: test_db_config['password'],
  port_bindings: { '5432': test_db_config['port'] },
  wait_for: :healthcheck
)