ci: allow --verbose flag to affect output of docker build (#9119)

`ci/cloudbuild/build.sh --docker --trigger=xxx --verbose` failed
to actually be verbose in the local build within the docker image
because the `--env=VERBOSE_FLAG=${VERBOSE_FLAG:-}` setting was
ignored in the `ci/cloudbuild/build.sh --local` reinvocation.
So, make the `VERBOSE_FLAG` initialization heed an existing value
in the environment.

Author: devbww

ci/cloudbuild/build.sh

@@ -105,7 +105,7 @@ CLEAN_FLAG="false"
 LOCAL_FLAG="false"
 DOCKER_FLAG="false"
 SHELL_FLAG="false"
-VERBOSE_FLAG="false"
+: "${VERBOSE_FLAG:=false}"
 while true; do
   case "$1" in
     --build)