I was trying to build coreutils the other day and kept running into an error with docbooks.
I’d initially figured out I was missing the docbook and docbook-xsl packages (fairly obvious given it said it couldn’t find it…), but having installed them, I then got this more cryptic error:
failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
Some searching and trawling through forum posts led me to try installing docbook-xml — this solved the problem.
tl;dr
Try installing docbook-xml via your distro’s package manager, a la;
sudo apt install docbook-xml