THIS IS NOT FOR RC. Temporary solution is to not use those images.

in qtbase/doc/global/qt-html-templates-offline.qdocconf

HTML.stylesheets += template/style/offline.css
HTML.extraimages += template/images/ico_out.png \
template/images/ico_note.png \
template/images/ico_note_attention.png \
template/images/btn_prev.png \
template/images/btn_next.png \
template/images/home.png \
template/images/arrow.png \
template/images/blu_dot.png \
template/images/bgrContent.png

Running 'make docs':

In file included from /home/jerome/gerrit/_qt5/qtbase/doc/global/qt-module-defaults.qdocconf:4,
/home/jerome/gerrit/_qt5/qtdoc/doc/config/qtdoc.qdocconf:1:
/home/jerome/gerrit/_qt5/qtbase/doc/global/qt-html-templates-offline.qdocconf:4: warning: Cannot open input file for copy: 'template/images/ico_out.png': No such file or directory

We agreed that we should use relative links and not the HTML.templatedir
The HTML.stylesheet variable works, but not extraimages