Add example of ENV variables for image and services.

What does this MR do?

Enhances documentation by providing a missing .gitlab-ci.yml example of passing ENV variables to Docker images and services. (which is only mentioned in 1 line on the variables doc page but has no examples.) Also adds reference back to Variables README.md for details.

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

N/A

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