Fix epub MWE building on Github CI

does not build the epub on my machine but that can be sorted later.

book/book.tex

@@ -68,7 +68,7 @@ produce a lot of \ch{CO2}.
 \end{figure}
 
 % Works
-% In this case generate the graphic with pdflatex first: cd figures && pdflatex fig-life-planet-sourdough-timeline-external.pdf
+% Generate first with: cd figures && pdflatex fig-life-planet-sourdough-timeline-external.pdf
 \begin{figure}[!htb]
   \includegraphics{figures/fig-life-planet-sourdough-timeline-external.pdf}
   \caption[Sourdough microbiology timeline]{Timeline works embedded as pdf}%

book/makefile

@@ -72,12 +72,19 @@ tgt_figures := $(patsubst %.tex, %.png,$(src_figures))
 %.png: %.tex
 	@echo "\input{./vars.tex}" > $@.in
 	# \b is backspace..
-	@echo "\\\begin{document}" >> $@.in
+	@echo "\\begin{document}" >> $@.in
 	@cat $< >> $@.in
-	@echo "\\\end{document}" >> $@.in
+	@echo "\\end{document}" >> $@.in
 	$(LATEX) $@.in
 	figures/export_figures.sh $@.pdf
 
+
+figures/fig-ethanol-oxidation-external.pdf: figures/fig-ethanol-oxidation-external.tex
+	$(LATEX) $<
+
+figures/fig-life-planet-sourdough-timeline-external.pdf: figures/fig-life-planet-sourdough-timeline-external.tex
+	$(LATEX) $<
+
 %.xbb: %.jpg
 	ebb -x $<
 
@@ -89,7 +96,10 @@ book_sans_serif/book_sans_serif.pdf: $(src_all)
 
 .PHONY: copy_ebook_files
 
-epub/%.epub: %.tex $(src_all) cover/cover-page.xbb
+epub/%.epub: %.tex $(src_all) cover/cover-page.xbb\
+	figures/fig-life-planet-sourdough-timeline-external.pdf\
+	figures/fig-ethanol-oxidation-external.pdf
+
 	$(EBOOK) $<
 
 copy_ebook_files: build_ebook