Add equivalent x-default feature from inspect's docker compose support

[craigwalton-dsit]

https://inspect.ai-safety-institute.org.uk/agents.html#multiple-environments

If you define multiple sandbox environments you are required to name one of them “default” so that Inspect knows which environment to resolve for calls to sandbox() without an argument. Alternatively, you can add the x-default key to a service not named “default” to designate it as the default sandbox.

We could do this with labels.