Merge pull request #154 from cisagov/improvement/update_pip3

Specify pip3 in the file where it got blown away by a rebase

Dockerfile

@@ -63,7 +63,7 @@ RUN apk --no-cache --quiet add ${DEPS}
 # cache.  This results in a smaller final image, at the cost of
 # slightly longer install times.
 ###
-RUN pip install --no-cache-dir --upgrade \
+RUN pip3 install --no-cache-dir --upgrade \
     pip \
     setuptools \
     wheel