Add the option of setting RUNTIME in the Conundrum build script

[Zawaken]

The part regarding the runtime is mostly just copy pasted from the docker_build.sh script in qmk_firmware

Description

Modified build-conundrum.sh to check if podman or docker container runtime exists on the machine, and pick the first it finds, and if none are found, error out.
Also added the possibility of using a commandline argument instead of environment variable to supply the custom keymap name.

Types of Changes

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

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project.
• 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).