Tell Git to use LF eol for .sh files

Without this, `vagrant up` fails on Windows:

    containerd-linux: /tmp/vagrant-shell: ./setup.sh: /bin/bash^M: bad interpreter: No such file or directory
The SSH command responded with a non-zero exit status. Vagrant
assumes that this means the command failed. The output for this command
should be in the log above. Please read the output to determine what
went wrong.

.gitattributes

@@ -0,0 +1 @@
+*.sh text eol=lf