Build custom keymap by specifying environment variable

[U47]

Easily specify a custom keymap by declaring a keymap environment variable with the name of the keymap which gets passed as a build arg to Docker

Description

Modified build-conundrum.sh to check for environment variable and pass it as a build-arg to Docker Modified Dockerfile to accept an ARG and populate the KEYMAP ENV with the ARGs value

Types of Changes

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

Issues Fixed or Closed by This PR

Checklist

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] 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).