[docker] Add missing typing-extensions dependency for pdfminer
pdfminer < 3.8 requires an extension: typing-extensions
: https://github.com/pdfminer/pdfminer.six/blob/617e4c8388fa61ed68b8ff450c48f79e41102be6/pdfminer/image.py#L10
Our docker images use node:16, which uses buster, which ships python 3.7.
Edited by Nicolas Frandeboeuf