Fixes a parsing issue where docker env var key/value pairs can contain
an "=" character in the value. This is pretty common and present in all
[nvidia/cuda](https://hub.docker.com/r/nvidia/cuda) docker images. Also
adds some tests for env var parsing.

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 a parsing failure where docker env var key/value pairs can
contain an "=" character in the value.