Docker images for Zephyr RTOS

Branches correspond to Zephyr releases.

Building the image

Simple! Just call make.

Using the image to build a Zephyr app

To build a Zephyr application, run:

docker run --rm -it brooksp/zephyr bash

then

cd /zephyrproject/zephyr
west build -p auto --board nrf52840dk_nrf52840 samples/basic/blinky