Add a script that simplifies running commands under docker.

[tzarc]

Description

Newer versions of clang-format produce different output compared to what's in the container.
Given that CI runs in the container, needed a simpler method of executing against the "blessed" version.

This adds a new command, util/docker_cmd.sh, which effectively just executes whatever arguments you pass to the script as another command, but inside the container.

Basically a dupe of util/docker_build.sh.

Types of Changes

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

Checklist

• My code follows the code style of this project: C, Python
• 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.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).