The missing `/etc/protocols` broke using images from the web. Install
`netbase`.

pdftk is now pdftk-java which drags in the JRE and makes the image even
bigger (and startup times longer). Use qpdf instead.

Useful for manipulating PDF files.

Also an example file for testing purposes and some Hebrew fonts.

Instead of using Alpine (which uses Musl) use Debian Buster.

- Use newer runas script, using gosu instead of sudo.
- Added test steps to the build using the example presentation (in fact
using Docker Hub as a sort of CI).
- Use newer Golang image.
- Small corrections to README.
- More streamlined Makefile.
- Added groff to image.
- Added XeTex to image.