Add support for userspace to docker build commands.

[tzarc]

Description

As per title. Both ./util/docker_build.sh and./util/docker_cmd.sh should be sorted, now. Simplified _build to call _cmd so there's no duplicate implementations.

Also adds two new hidden CLI commands -- qmk path and qmk userspace-path to print out the computed values for both qmk_firmware and qmk_userspace.

Types of Changes

• [ ] Core
• [ ] Bugfix
• [x] New feature
• [x] Enhancement/optimization
• [ ] Keyboard (addition or update)
• [ ] Keymap/layout/userspace (addition or update)
• [ ] Documentation

Issues

• Fixes #18130 as well.
• Fixes https://github.com/qmk/qmk_userspace/issues/24
• Fixes https://github.com/qmk/qmk_firmware/issues/23984

Checklist

• [x] My code follows the code style of this project: C, Python
• [x] I have read the PR Checklist document and have made the appropriate changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).