Docker Build eBooks have character encoding problem

[entorb]

running

docker run -it --mount type=bind,src="$(pwd)",dst=/app hpmor scripts/make_ebooks.sh

the resulting hpmor.html and .epub have broken umlaute in on the titlepage.

Compiling via GitHub Actions, they are fine.

[entorb]

Origin is

\usepackage[bookmarks=true,unicode=true,pdfborder={0 0 0},
	pdftitle={Harry Potter and the Methods of Rationality - Deutsche Fan-Übersetzung},
	pdfauthor={Eliezer Yudkowsky}, breaklinks={true},
	pdfkeywords={Harry Potter, rationality, hpmor},pdfencoding=auto
]{hyperref}