Try to fix Docker caching

Dockerfile

@@ -34,27 +34,14 @@ RUN \
 
 # Install any needed packages specified in requirements.txt
 # RUN pip install --trusted-host pypi.python.org -r requirements.txt
-# COPY requirements.txt /tmp/
-RUN pip install \
-    # Program related
-    pyyaml>=5.1.1 \
-    cerberus>=1.3.1 \
-    pytest>=5.0.1 \
-    pytest-cov>_2.7.1 \
-    Sphinx>=2.1.2 \
-    sphinxcontrib-napoleon>=0.7 \
-    Pygments>=2.4.2 \
-    # IDE related (can be skipped)
-    pylint>=2.3.1 \
-    rope>=0.14.0 \
-    doc8>=0.8.0 \
-    autopep8>=1.4.4 \
-    pydocstyle>=4.0.1
+WORKDIR /fenicsR13
+ADD ./requirements.txt /fenicsR13/requirements.txt
+RUN pip install -r /fenicsR13/requirements.txt
 
 # Install the fenicsR13 package (puts it into the PATH)
-COPY . /tmp/
-RUN pip install --editable /tmp/.
+ADD . /fenicsR13
+RUN pip install --editable /fenicsR13/.
 
 # Replace default FEniCS Docker WELCOME screen with custom WELCOME screen
-COPY WELCOME .
+ADD WELCOME .
 RUN echo "Built: $(date)" >> WELCOME

requirements.txt

 # Program related
+six>=1.12.0
 pyyaml>=5.1.1
 cerberus>=1.3.1
 pytest>=5.0.1
-pytest-cov>_2.7.1
+pytest-cov>=2.7.1
 Sphinx>=2.1.2
 sphinxcontrib-napoleon>=0.7
 Pygments>=2.4.2
@@ -12,4 +13,4 @@ pylint>=2.3.1
 rope>=0.14.0
 doc8>=0.8.0
 autopep8>=1.4.4
-pydocstyle>=4.0.1
\ No newline at end of file
+pydocstyle>=4.0.1