tweak Dockerfile

Dockerfile

 FROM debian:testing-slim
 
 RUN apt-get update && apt-get install -y \
-    # Needed for gitlab tests
-    git \
+    # Needed for setup
+    git python3-pip \
     # Packages needed for NIFTy
-    libfftw3-dev \
-    python3 python3-pip python3-scipy \
+    python3-scipy \
     # Documentation build dependencies
     python3-sphinx-rtd-theme \
     # Testing dependencies
-    python3-pytest-cov \
+    python3-pytest-cov jupyter \
     # Optional NIFTy dependencies
-    python3-mpi4py \
-    # Packages needed for NIFTy
+    libfftw3-dev python3-mpi4py python3-matplotlib \
+  # more optional NIFTy dependencies
   && pip3 install pyfftw \
-  # Optional NIFTy dependencies
   && pip3 install git+https://gitlab.mpcdf.mpg.de/ift/pyHealpix.git \
-  # Testing dependencies
-  && rm -rf /var/lib/apt/lists/*
-
-# Needed for demos to be running
-RUN apt-get update && apt-get install -y python3-matplotlib \
-  && python3 -m pip install --upgrade pip && python3 -m pip install jupyter \
+  && pip3 install jupyter \
   && rm -rf /var/lib/apt/lists/*
 
 # Set matplotlib backend