Update Dockerfile and explain why

Author: jdutant

.tools/Dockerfile

@@ -1,7 +1,7 @@
 FROM pandoc/latex:latest
 
 # Currently pandoc uses TexLive 2022, need to point there
-RUN tlmgr option repository https://ftp.tu-chemnitz.de/pub/tug/historic/systems/texlive/2022/tlnet-final \
+RUN tlmgr option repository https://ftp.tu-chemnitz.de/pub/tug/historic/systems/texlive/2024/tlnet-final \
     && tlmgr install \
     standalone \
     dvisvgm

README.md

@@ -77,6 +77,15 @@ Installation and usage
 
 See the [manual][ImagifyManual].
 
+CI Tests
+--------
+
+CI tests run on the
+[pandoc/latex][https://hub.docker.com/r/pandoc/latex] Docker image.
+The [Dockerfile](.tools/Dockerfile) installs two LaTeX packages not
+included in that image and needs to point out the the same version
+of TeXLive. 
+
 Issues and contributing
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