Try to create more slim Docker layers

Dockerfile

 FROM debian:testing-slim
 
-RUN apt-get update
-
-# Needed for gitlab tests
-RUN apt-get install -y git
-
-# Packages needed for NIFTy
-RUN apt-get install -y libfftw3-dev
-RUN apt-get install -y python python-pip python-dev python-future python-scipy
-RUN apt-get install -y python3 python3-pip python3-dev python3-future python3-scipy
-RUN pip install pyfftw
-RUN pip3 install pyfftw
-
-# Optional NIFTy dependencies
-RUN apt-get install -y openmpi-bin libopenmpi-dev python-mpi4py python3-mpi4py
-RUN pip install git+https://gitlab.mpcdf.mpg.de/ift/pyHealpix.git
-RUN pip3 install git+https://gitlab.mpcdf.mpg.de/ift/pyHealpix.git
-
-# Documentation build dependencies
-RUN apt-get install -y python-sphinx python-sphinx-rtd-theme python-numpydoc
-
-# Testing dependencies
-RUN apt-get install -y python-nose python-parameterized
-RUN apt-get install -y python3-nose python3-parameterized
-RUN pip install coverage
+RUN apt-get update && apt-get install -y \
+    # Needed for gitlab tests
+    git \
+    # Packages needed for NIFTy
+    libfftw3-dev \
+    python python-pip python-dev python-future python-scipy \
+    python3 python3-pip python3-dev python3-future python3-scipy \
+    # Documentation build dependencies
+    python-sphinx python-sphinx-rtd-theme python-numpydoc \
+    # Testing dependencies
+    python-nose python-parameterized \
+    python3-nose python3-parameterized \
+    # Optional NIFTy dependencies
+    openmpi-bin libopenmpi-dev python-mpi4py python3-mpi4py \
+    # Packages needed for NIFTy
+  && pip install pyfftw \
+  && pip3 install pyfftw \
+  # Optional NIFTy dependencies
+  && pip install git+https://gitlab.mpcdf.mpg.de/ift/pyHealpix.git \
+  && pip3 install git+https://gitlab.mpcdf.mpg.de/ift/pyHealpix.git \
+  # Testing dependencies
+  && pip install coverage \
+  && rm -rf /var/lib/apt/lists/*
 
 # Create user (openmpi does not like to be run as root)
 RUN useradd -ms /bin/bash testinguser