dolos: add Dockerfile with builder

This image may be used to build firmware-zephyr with recent
upstream changes to the Zephyr

BUG=b:373935038
TEST=Remove old firmware-zephyr/build directory
     Create docker image:
     $ docker build -f dockerfiles/Dockerfile.builder -t dolos .
     Run container to build the firmware:
     $ docker run --rm --user $(id -u):$(id -g) -v `pwd`:/repo -it dolos
     Flash built firmware:
     $ fw-updater/target/debug/fw-updater firmware-zephyr/build/zephyr.txt

Change-Id: Ibaddf0dd1234567890533e8dbf500fd90b73fa8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dolos/+/6075085
Reviewed-by: Łukasz Hajec <hajec@google.com>
Tested-by: Brian Nemec <bnemec@google.com>
Reviewed-by: Brian Nemec <bnemec@google.com>
Commit-Queue: Michał Headcrab Barnaś <barnas@google.com>
Reviewed-by: Keith Haddow <haddowk@chromium.org>
2 files changed