Improve docker build speed on multicore machines (#2148)

docker/node/Dockerfile

@@ -3,11 +3,13 @@ FROM nanocurrency/nano-env:gcc
 ARG NETWORK=live
 ADD ./ /tmp/src
 
+RUN num_cores=$(($(grep -c ^processor /proc/cpuinfo) + 1))
+
 RUN mkdir /tmp/build && \
     cd /tmp/build && \
     cmake /tmp/src -DBOOST_ROOT=${BOOST_ROOT} -DACTIVE_NETWORK=nano_${NETWORK}_network && \
-    make nano_node && \
-    make nano_rpc && \
+    make nano_node -j $num_cores && \
+    make nano_rpc -j $num_cores && \
     cd .. && \
     echo ${NETWORK} > /etc/nano-network