Fix CI error

The error is due to the docker image. This changes the image to one that works and seems to be better maintained.

spec/doc.tex

@@ -5,9 +5,10 @@
 \documentclass{scrartcl}
 
 \usepackage{scrlayer-scrpage}
-\usepackage{unicode-math}
 \usepackage{fontsetup}
+\setmainfont{CMU Serif}
 
+\usepackage{unicode-math}
 \usepackage{minted}
 \usemintedstyle{trac}
 \renewcommand{\MintedPygmentize}{./highlight.py}