Just in case anyone needs to run some custom code for fastpages convert, but the required package is not available in the docker-image. You don’t need to edit Dockerfile, that would not work except locally.
What you need to do is to install everything you need in line 8, e.g. pip install pandas --user
Of course, it should be your blog repository, not fastpages main repo.
For my case, it was a custom version of nbdev I needed, but it can be any pip/pip+github package.
Hope, it saves some time for someone 