Fix Container Scanning job with K8S runners

What does this MR do?

Fix the Container Scanning job timing out when using gitlab-runner with the kubernetes executor.

The test [ -z "$DOCKER_HOST" -a "$KUBERNETES_PORT" ] taken from auto-devops is always false, since we define DOCKER_HOST all the time. This MR is fixing this behavior.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11448