Apply isolation config to service containers (closes #39620)

What does this MR do?

Applies the Docker executor isolation config option (e.g. process, hyperv) to service containers, matching existing behavior for the build container.

Why was this MR needed?

Service containers ignored the configured isolation mode and always used the Docker daemon default. Other config properties like runtime and userns-mode were already honored for both container types — isolation was simply overlooked.

What's the best way to test this MR?

Run a Docker executor job with isolation = "hyperv" on Windows. Service containers should now also use Hyper-V isolation.

What are the relevant issue numbers?

Closes #39620 (closed)