FROM scratch is a completely empty filesystem. You have no installed libraries, and no shell (like /bin/sh) included in there. To use this as your base, you’d need a statically linked binary, or you’ll need to install all of the normal tools that are included with a linux distribution.
The latter is what is prepackaged in the various busybox, debian, ubuntu, centos, etc images on the docker hub. The fast way to make your image work with a minimal base image is to change the from to FROM busybox and change your /bin/bash to /bin/sh.