Add default values for ENV

login_provider/test/behave/Dockerfile

 ARG baseimage=open.greenhost.net:4567/openappstack/single-sign-on/login_provider
 FROM $baseimage
 
-ENV TESTUSER_USERNAME
-ENV TESTUSER_EMAIL
-ENV TESTUSER_PASSWORD
+ENV TESTUSER_USERNAME test
+ENV TESTUSER_EMAIL test@example.net
+ENV TESTUSER_PASSWORD test
 
 COPY mock/db.py db.py