Use docker for Zephyr CI

examples/zephyr/http-server/Makefile

@@ -1,13 +1,14 @@
-CWD = $(realpath $(CURDIR))
-ZEPHYR_DIR ?= $(realpath ../../../../../../zephyrproject)
+PROJECT_DIR ?= /mnt
+ZEPHYR_DIR ?= /zephyrproject
 BOARD ?= nucleo_f746zg
+DOCKER ?= docker run -v $(realpath $(CURDIR)):/mnt mdashnet/zephyr
 
 example:
 	true
 
 build:
 	cp ../../../mongoose.c ../../../mongoose.h src/
-	cd $(ZEPHYR_DIR) && west build -b $(BOARD) -p auto $(CWD)
+	$(DOCKER) /bin/sh -c 'cd $(ZEPHYR_DIR) && west build -b $(BOARD) -p auto $(PROJECT_DIR)'
 
 flash:
 	cd $(ZEPHYR_DIR) && west flash