use containerWorkspaceFolder var and pass it to setup.sh (#12)

- folder name inside dev container is same as folder name on host
  so if it differs from qmk_userspace then setup.sh cannot be found
  and it sets incorrect user.overlay_dir
- docs https://containers.dev/implementors/json_reference/

Replicate qmk/qmk_firmware#23310

Add support for devcontainers. (#6)

Point workflows at `qmk/.github`s `main` branch (#1)

Updated `.gitignore`.

Initial structure.