Add troubleshooting entry on private container registries

Problem

When configuring DAST API/API Fuzzing, many times the user will deploy the target application using a services: entry on the dast_api job. If the customer is pulling from a private registry that requires authentication, the job will fail if they have not set authentication into a special CI variable.

Example customer issue: https://gitlab.slack.com/archives/CKWJP0ZS7/p1661371924147489

Article with solution: https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#access-an-image-from-a-private-container-registry

Document this issue including a work around of adding the container to the projects gitlab registry if the authentication isn't possible for some reason. Also add search terms:

• Cloud provider names (Azure, Google Cloud (GCP), AWS, Digital Ocean)
• Failure string when unable to pull due to auth
• ???

Proposal

Keeping in mind that the focus is the show the user how to consume a goal is to show the user how to pull a a target image from registry that requires authorization.

1. [-] Set up a demo project within GitLab which exemplifies how to access a target image from a private Container Registry (one for API Fuzzing and one for DAST API)
2. Update API Fuzzing and DAST API documentation to
   1. Include a link to the example Added sample instead
   2. Include a section (or troubleshoot section) about how to handle this scenario
      1. Explain how to use the supported authentication method, link to example
      2. Explain the workaround about using GitLab registry
   3. Include a list of requirements for the target.