tools/ci.sh: Upgrade Zephyr docker image to v0.21.0.

As a prerequisite to upgrading to Zephyr v2.7.0, upgrade CI to use
Zephyr docker image v0.21.0. In particular, this is needed to pick up a
newer CMake version because Zephyr v2.7.0 increased the minimum CMake
version required to 3.20.0.
Signed-off-by: Maureen Helm <maureen.helm@intel.com>

tools/ci.sh

@@ -603,15 +603,18 @@ function ci_windows_build {
 ########################################################################################
 # ports/zephyr
 
+ZEPHYR_DOCKER_VERSION=v0.21.0
+ZEPHYR_SDK_VERSION=0.13.2
+
 function ci_zephyr_setup {
-    docker pull zephyrprojectrtos/ci:v0.17.3
+    docker pull zephyrprojectrtos/ci:${ZEPHYR_DOCKER_VERSION}
     docker run --name zephyr-ci -d -it \
       -v "$(pwd)":/micropython \
-      -e ZEPHYR_SDK_INSTALL_DIR=/opt/toolchains/zephyr-sdk-0.12.4 \
+      -e ZEPHYR_SDK_INSTALL_DIR=/opt/toolchains/zephyr-sdk-${ZEPHYR_SDK_VERSION} \
       -e ZEPHYR_TOOLCHAIN_VARIANT=zephyr \
       -e ZEPHYR_BASE=/zephyrproject/zephyr \
       -w /micropython/ports/zephyr \
-      zephyrprojectrtos/ci:v0.17.3
+      zephyrprojectrtos/ci:${ZEPHYR_DOCKER_VERSION}
     docker ps -a
 }