Dockerfile: Fix DOS EOL on shell scripts

[jayvdb]

If the repo is checked out with git "autocrlf" enabled, the DOS EOLs cause the scripts to not be executable. https://willi.am/blog/2016/08/11/docker-for-windows-dealing-with-windows-line-endings/ explains it.

I'm happy to move dos2unix into the relevant RUN command and purge it at the end of the RUN. I havent done that as this is a tiny program.