Update config.yml

Author: EarthmanT

.circleci/config.yml

@@ -97,7 +97,7 @@ commands:
           name: Build RHEL 27 Wagon
           command: |
               git clone https://github.com/cloudify-cosmo/cloudify-wagon-build-containers.git
-              docker build -t cloudify-redhat-7-wagon-builder cloudify-wagon-build-containers/redhat_7 $USERNAME $PASSWORD
+              docker build -t cloudify-redhat-7-wagon-builder cloudify-wagon-build-containers/redhat_7 --build-arg USERNAME="$USERNAME" --build-arg PASSWORD="$PASSWORD"
               docker run -v ~/project/:/packaging cloudify-redhat-7-wagon-builder
       - run:
           name: copy wagon to workspace