.github: switch docker image from ':latest' to ':v0.17.3'

Even though "latest" is being referenced by the current
https://github.com/zephyrproject-rtos/docker-image/tree/072880b3#readme
I've been told it's too "bleeding edge" to be used in CI.

v0.17.3 also has libssl-dev which was the only thing we were missing.

Signed-off-by: Marc Herbert <[email protected]>

Author: marc-hb

.github/workflows/zephyr.yml

@@ -20,5 +20,5 @@ jobs:
 
       - name: build
         run: docker run -v "$(pwd)":/workdir
-             docker.io/zephyrprojectrtos/zephyr-build:latest
+             docker.io/zephyrprojectrtos/zephyr-build:v0.17.3
              ./zephyr/docker-build.sh