.github: add Zephyr build

Use the official image from the Zephyr project
https://github.com/zephyrproject-rtos/docker-image

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

.github/workflows/zephyr.yml

@@ -0,0 +1,24 @@
+---
+
+name: Zephyr
+
+# 'workflow_dispatch' allows running this workflow manually from the
+# 'Actions' tab
+# yamllint disable-line rule:truthy
+on: [push, pull_request, workflow_dispatch]
+
+jobs:
+  zephyr-build:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+        # From time to time this will catch a git tag and change SOF_VERSION
+        with: {fetch-depth: 10, submodules: recursive}
+
+      - name: yamllint ourselves
+        run: yamllint .github/workflows/zephyr.yml
+
+      - name: build
+        run: docker run -v "$(pwd)":/workdir
+             docker.io/zephyrprojectrtos/zephyr-build:latest
+             ./zephyr/docker-build.sh

zephyr/docker-build.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+# SPDX-License-Identifier: BSD-3-Clause
+# Copyright(c) 2021 Intel Corporation. All rights reserved.
+
+# "All problems can be solved by another level of indirection"
+# Ideally, this script would not be needed.
+
+set -e
+set -x
+
+# export http_proxy=...
+# export https_proxy=...
+
+unset ZEPHYR_BASE
+
+sudo --preserve-env=http_proxy,https_proxy apt-get update
+sudo --preserve-env=http_proxy,https_proxy apt-get -y install libssl-dev
+
+# Make sure we're in the right place; chgrp -R below.
+test -e ./scripts/xtensa-build-zephyr.sh
+
+if test -e zephyrproject; then
+    ./scripts/xtensa-build-zephyr.sh -a
+else
+    # Matches docker.io/zephyrprojectrtos/zephyr-build:latest gid
+    ls -ln | head
+    stat .
+    sudo chgrp -R 1000 .
+    sudo chmod -R g+rwX .
+    ./scripts/xtensa-build-zephyr.sh -a -c
+fi