ci: bump ci image to 0.6

Sync with other zephyr components on 0.6

Signed-off-by: Kumar Gala <[email protected]>

Author: galak

.shippable.yml

@@ -23,7 +23,7 @@ build:
         - ${SHIPPABLE_BUILD_DIR}/sources
     pre_ci_boot:
         image_name: zephyrprojectrtos/ci
-        image_tag: v0.5-rc5
+        image_tag: v0.6
         pull: true
         options: "-e HOME=/home/buildslave --privileged=true --tty --net=bridge --user buildslave"