When a Docker Compose service specifies a build context, the generated
override file was replacing it with an empty context directory. This
meant Dockerfiles that reference files relative to their build context
(e.g. `COPY . /app`) would fail. The fix preserves the original build
context from the compose service, falling back to the empty context
directory only when no context was specified.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] 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 docker compose Dockerfile build context not being preserved in
dev_container integration.

---------

Co-authored-by: KyleBarton <kjb@initialcapacity.io>