Docker: Fix PDF library & viewing

[ryelle]

I was having trouble with the PDF generation functionality, and it looks like the wkhtmltopdf library wasn't loaded correctly. I'm not entirely sure updating this in the docker is correct (or if this was just an issue with my install), but this fixed the issues for me.

When generating an invoice, I hit this error:

wkhtmltopdf: error while loading shared libraries: libXrender.so.1: cannot open shared object file: No such file or directory

So this PR updates the docker setup to install the Debian 9 (stretch) version of wkhtmltopdf, which does not rely on linked resources (see the note about generic builds). I also updated the nginx configuration to serve .pdf files from the uploads directory.

Note: I'm not sure that Debian 9 is actually the correct OS- it's what I've got, but it looks like the php:7.2-fpm base should be installing Debian 10.

To test

1. Try generating a doc or a ticket invoice
2. It should work 🎉
3. You should be able to load the invoice from the site

(not requesting reviews since I haven't take the time to test a clean install yet, but anyone should feel free to comment 🙂 )

[timiwahalahti]

Just tested this, and it worked when the URL of the package was changed. It would be nice to get this merged and Docker env also working on this part!

[timiwahalahti]

WordCamp Docs also need libxrender1 for wkhtmltopdf to work. So adding apt-get install libxrender1 is required.