Project contents:
rob@work:~/git/proj $ ls
lib node_modules props.json start.sh
app.js Dockerfile package.json README.md
You have some issues:
You must use ./start.sh to run the start.sh file from current directory. /start.sh run start.sh in root /, which does not exist.
Your shebang line in start.sh script is wrong, it's must be #!/bin/bash.
You also must set executable permission for start.sh, by running chmod +x start.sh.