contrib: Add pytest-reruns into the builder image

Should allow us to reduce the number of manual reruns, but may hide
some flaky tests.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

contrib/Dockerfile.builder

@@ -35,4 +35,5 @@ RUN cd /tmp/ && \
     mv /tmp/bitcoin-0.15.1/bin/bitcoin* /usr/local/bin/ && \
     rm -rf bitcoin.tar.gz /tmp/bitcoin-0.15.1
 
-RUN pip3 install python-bitcoinlib==0.7.0 pytest==3.0.5 setuptools==36.6.0 pytest-test-groups==1.0.3 flake8==3.5.0
+RUN pip3 install --upgrade pip && \
+    pip3 install python-bitcoinlib==0.7.0 pytest==3.0.5 setuptools==36.6.0 pytest-test-groups==1.0.3 flake8==3.5.0 pytest-rerunfailures==3.1

contrib/Dockerfile.builder.i386

@@ -35,4 +35,5 @@ RUN cd /tmp/ && \
     mv /tmp/bitcoin-0.15.1/bin/bitcoin* /usr/local/bin/ && \
     rm -rf bitcoin.tar.gz /tmp/bitcoin-0.15.1
 
-RUN pip3 install python-bitcoinlib==0.7.0 pytest==3.0.5 setuptools==36.6.0 pytest-test-groups==1.0.3 flake8==3.5.0
+RUN pip3 install --upgrade pip && \
+    pip3 install python-bitcoinlib==0.7.0 pytest==3.0.5 setuptools==36.6.0 pytest-test-groups==1.0.3 flake8==3.5.0 pytest-rerunfailures==3.1