Fix the loop for pulling images

When starting with loop=0, ((loop++)) actually returns 0, which is interpretted as false and thus the loop does not happen.

build.sh

@@ -105,7 +105,7 @@ function pull_image {
     # Try pulling the image to see if it is accessible
     # WORKAROUND: Since Fedora registry sometimes fails randomly, let's try it more times
     while ! docker pull "$image_name"; do
-      ((loop++))
+      ((loop++)) || :
       echo "Pulling image $image_name failed."
       [ "$loop" -gt "$loops" ] && { echo "It happened $loops times. Giving up." ; return 1; }
       echo "Let's wait $((loop*5)) seconds and try again."