Include `luarocks` in images

[tarleb]

Luarocks is a package manager for Lua. The Lua packages provided by OS packages sometimes don't match the Lua version included in pandoc, so they cannot be used in pandoc scripts. Installing packages via luarocks could fix that.

[alerque]

Yes, I'm in favor of this. The system default Luarocks config should be setup to match Pandoc's internal Lua version by default too. (In fact it could even be set to use Pandoc as its default Lua interpreter, although that might not gain anybody anything.)