Gitea has some good docs on how to do this:

https://docs.gitea.io/en-us/external-renderers/#installing-external-binaries

I was able to get this working on my existing instance by:

mkdir /app/data/python_packages
cd /app/data/python_packages
python3 -m virtualenv .
source bin/activate
pip3 install jupyter

And then adding the following to my /app/data/app.ini file:

[markup.jupyter]
ENABLED = true
FILE_EXTENSIONS = .ipynb
RENDER_COMMAND = "/app/data/python_packages/bin/jupyter nbconvert --stdin --stdout --to html --template basic"
IS_INPUT_FILE = false

[markup.sanitizer.jupyter.img]
ALLOW_DATA_URI_IMAGES = true

I think all you'd have to add to the Dockerfile is something like:

RUN pip3 install jupyter

And then make the changes to the app.ini.template (without the path prefix on the render command, since it would be in the path now).

I'd be happy to submit and test a pull request against the repository if that would be easiest.

While at it...could probably also add pandoc so that we can add our own custom markup types for anything pandoc supports.