The `md2pdf` bash script created during the Docker image build was
invalid. For some reason, it was scrapping out the line break:
```
root@92820d4e9bc1:/resume# cat /usr/bin/md2pdf
```
As a result `md2resume` was failing silently. Also, it was not possible
to run wkhtmltopdf standalone headlessly inside the container. These
changes allow wkhtmltopdf to be run headlessly and fix the issue with
`md2resume pdf`.
Matheus Marchini
spawnia