Update warning for alias overlap in executors/docker/services.go

What does this MR do?

This MR modifies the warning for when two services aliases overlap (eg, two docker services use 'nginx'). We currently provide a misleading warning to the user which makes it seem like the services failed to spin up, when in reality, the aliases failed to apply because they overlap.

Closes #38756 (closed)

Why was this MR needed?

To make the actual cause and implication of the issue (alias overlap, alias cannot be applied) clear to the user/developer.

What's the best way to test this MR?

Tests were added as part of the MR.

For manual testing:

• Build the binary: mage build:local (or go build .)
• Register a local runner against a test project: ./gitlab-runner register
• Run it: ./gitlab-runner run
• Push a .gitlab-ci.yml matching the ticket (two nginx:latest services with aliases abc/def)
• Check the job log shows the new warning text and that both services come up

What are the relevant issue numbers?

Closes #38756 (closed)