As promised, what I found so far: the following dockerfile yeilds a usable cross-compile environment, provided you use it with a Makefile.

FROM ubuntu:xenial

RUN apt-get update \
    && apt-get install --yes \
      vim \
      build-essential \
      git

RUN git clone https://github.com/raspberrypi/tools

RUN git clone https://gist.github.com/3873805.git /build

WORKDIR "/build"

CMD ["/usr/bin/make", "CC=/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin/arm-linux-gnueabihf-gcc"
, "HelloWorld"]


# Seems I need a real makefile for the above to work.

For cross compiling mysensors to work, hoever, you also need to properly seed the variable in configure, for it to work correctly. This is where I left off.

Sources:
https://stackoverflow.com/questions/18007326/how-to-change-default-values-of-variables-like-cc-in-makefile
https://bitbucket.org/mitchallen/pi-hello-cross-compile/src/master/
https://desertbot.io/blog/how-to-cross-compile-for-raspberry-pi
https://github.com/mitchallen/pi-cross-compile/blob/master/Dockerfile
https://github.com/mysensors/MySensors/blob/development/configure
https://github.com/raspberrypi/tools/tree/master/arm-bcm2708/arm-bcm2708-linux-gnueabi/bin
https://www.raspberrypi.org/documentation/linux/kernel/building.md
https://www.mysensors.org/build/raspberry

@monte yeah, it does only take that long. I think I got sidetracked a bit 😅 I usually set up my pi by Ansible, but it's very slow. I was thinking about ways to speed that up.

Maybe get my project working first before I dive into this. I'll post what I have so far shortly should anyone be interesed.

Thanks for screwing my head on straight again 😁