Fix error message about missing Python exec

Message is
unable to start container process: exec: "python": executable file not found in $PATH: unknown.

Closes #1260

.docker/Dockerfile

@@ -64,6 +64,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   img2pdf \
   libsm6 libxext6 libxrender-dev \
   pngquant \
+  python-is-python3 \
   tesseract-ocr \
   tesseract-ocr-chi-sim \
   tesseract-ocr-deu \