Non-reproducible container build failure with meson and gtk-doc

After !147 (merged), I had a crash that the logs don't say much about except that there is an attempt to copy files that have not yet been generated:

#10 157.5 cp: cannot stat 'build/docs/reference/libxfce4windowing/html/.': No such file or directory
#10 ERROR: process "/bin/sh -c /git/build_libs.sh" did not complete successfully: exit code: 1

Relaunching the build fixes the problem:

#10 182.6 Running custom install script '/usr/bin/meson --internal gtkdoc --sourcedir=/git/libxfce4windowing --builddir=/git/libxfce4windowing/build

I don't think it's related to libxfce4windowing, in my opinion it could just as easily happen with any other project containing docs. And I don't think we're doing anything wrong here either, so I wouldn't be surprised if the problem was with meson or the GNOME module (for which we've already reported an unnecessary doc rebuild bug in the past).

So I wonder if we shouldn't just remove the doc from the container build, since it serves no purpose I think, and just leave it in CI.