utils: fix permissions for auto doc updates

it's required to change owner of the WORKDIR with the help of
'prepare-for-build.sh' script.

Author: lukaszstolarczuk

utils/docker/run-doc-update.sh

@@ -10,6 +10,8 @@
 
 set -e
 
+source $(dirname ${0})/prepare-for-build.sh
+
 if [[ -z "${DOC_UPDATE_GITHUB_TOKEN}" ]]; then
 	echo "To build documentation and upload it as a Github pull request, variable " \
 		"'DOC_UPDATE_GITHUB_TOKEN' has to be provided."