You cant update pythonpath inside a running notebook, as this has to be done prior to running the container. An example of how to do this is shown below (spawning the complex build of PETSc/dolfin with gmsh)
docker run -v $(pwd):/root/shared -w "/root/shared" --rm --env LD_LIBRARY_PATH=/usr/local/dolfinx-complex/lib --env PATH=/usr/local/dolfinx-complex/bin:/usr/local/gmsh-4.6.0-Linux64-sdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin --env PKG_CONFIG_PATH=/usr/local/dolfinx-complex/lib/pkgconfig --env PETSC_ARCH=linux-gnu-complex-32 --env PYTHONPATH=/usr/local/dolfinx-complex/lib/python3.8/dist-packages:/usr/local/gmsh-4.6.0-Linux64-sdk/lib -p 8888:8888 dolfinx/lab