CI + CD builds nginx image during API up

[dhruvkb]

Problem

The nginx image is always built in CI job "Run tests for the API". This is because the api/compose.yml does not have an image field in the nginx service, which forces it to be built when API is brought up.

• https://github.com/WordPress/openverse/actions/runs/8992601250/job/24702911944#step:5:13
• https://github.com/WordPress/openverse/actions/runs/8989040826/job/24691377170#step:5:9

Description

Add the field services.nginx.image to ensure that the image is built-once and cached and subsequently re-used.

Alternatives

Another alternative is to drop nginx and proxy from the API profile and let them be started manually by the developers who actually need to run them.