For docker-compose-based dev containers, build args live on the primary
compose service's `build.args`, not on `dev_container.build` (which is
`None` in the compose case). `expanded_dockerfile_content` only
consulted the latter, so `${…}` references in the service's Dockerfile —
e.g. `FROM ${BASE_IMAGE}` — were never substituted. Downstream callers
then invoked `docker inspect "${BASE_IMAGE}"` and `docker pull
"${BASE_IMAGE}"`, both of which fail and the dev container fails to
start.

A new unit test `test_expands_compose_service_args_in_dockerfile` mounts
a Dockerfile with `FROM ${BASE_IMAGE}` backed by a compose service whose
`build.args` define `BASE_IMAGE=test_image:latest`, and asserts both
`expanded_dockerfile_content` and `image_from_dockerfile` produce the
resolved reference.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed dev container startup failing for docker-compose configs whose
service Dockerfile uses build-arg substitution in the FROM line (for
example, `FROM ${BASE_IMAGE}`).