Refs #37375 - Refactor ContainerGatewayBackendTest to a simple unit test
The ContainerGatewayMain class doesn't need anything from the API, so this strips away all the API parts that aren't needed. It now uses simple constructor arguments instead of mocking out any test settings.
Related issues
Refactor #37375: Change container_gateway to only accept a database connection string
Refs #37375 - Refactor ContainerGatewayBackendTest to a simple unit test
The ContainerGatewayMain class doesn't need anything from the API, so
this strips away all the API parts that aren't needed. It now uses
simple constructor arguments instead of mocking out any test settings.