Fix STI builds when using host docker

The issue with host docker is that the sti build is trying to mount directories from the /tmp directory in the container it runs. Because the /tmp directory is from the container and not the host, the expected content is not there. The solution is to mount the /tmp directory of the host onto the sti builder container. Because we are now mounting this /tmp directory, I relocated the build.sh to /opt instead of /tmp in the STI builder image.