Set minimum QBS version in native images

The following discussion from clickable-docker-images!102 (merged) should be addressed:

• @jonnius started a discussion: (+3 comments)

  Please do the same change to the native docker image to keep consistency. To do so I'd change the QBS_SUPPORT variable in the build.sh and in the Dockerfile I'd change

  RUN if [ "$QBS_SUPPORT" = "qbs" ]; then \

  to check for not empty

  RUN if [ ! -z "$QBS_SUPPORT" ]; then \