Force Unix line endings on shell scripts

When following the quickstart docker instructions on Windows (using
Docker for Windows), an error is encountered when running the netbox
container. This is caused by git converting the line endings of the
docker-entrypoint.sh script to Windows format, which are then copied
into the container image.

By setting .gitattributes, we force LF rather than CRLF line endings on
shell scripts on Windows. Other files are left as is.

.gitattributes

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