Update using_docker_images.md to explicitly note that default environment...

What does this MR do?

Recently when trying to get a linked service container to leverage information provided as environment variables, it became clear that only explicitly provided environment variables are passed along to linked containers. And that this does not include variables available by 'default' as provided by Gitlab CI

I'm hoping to clarify things for other folks who might run into this issue and be as confused as I was.

What are the relevant issue numbers?

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer