Adding pdfunite to Image

Adding pdfunite to the image to use this tool instead of Ghostscript to merge PDF-files. Because GS causes problems for Mozilla Firefox to display the PDFs.

.devcontainer/image/Dockerfile

@@ -81,6 +81,9 @@ RUN apt-get update && \
  python3-pygments \
  # Required to embed git metadata into PDF from within Docker container:
  git
+ # Required to use pdfunite for merging PDF files. Ghostscript is causing
+ # trouble with Mozilla Firefox's PDF Reader.
+ poppler-utils
 
 # The `minted` LaTeX package provides syntax highlighting using the Python `pygmentize`
 # package. That package also installs a callable script, which `minted` uses, see