Build CircuitPython in a Github-Devcontainer

To build CircuitPython within a Github-Devcontainer, you need to perform the following steps.

checkout the code to a devcontainer

- click on the green "<> Code"-button
- select the Codespaces-tab
- choose "+ new with options..." from the "..."-menu
- in the following screen select the branch and then
- select ".devcontainer/cortex-m/devcontainer.json" instead
  of "Default Codespaces configuration"
- update region as necessary
- finally, click on the green "Create codespace" button

Your codespace is created. Cloning the images is quite fast, but preparing it for CircuitPython-development takes about 10 minutes. Note that this is a one-time task.
During creation, you can run the command tail -f /workspaces/.codespaces/.persistedshare/creation.log to see what is going on.

To actually build CircuitPython, run

cd ports/raspberrypi
make -j $(nproc) BOARD=whatever TRANSLATION=xx_XX

This takes about 2m40s.