pip not pip3, works then in Py2.x.

Author: javabrett

Dockerfile

@@ -3,9 +3,9 @@ FROM python:3.6
 ADD . /flasgger
 WORKDIR /flasgger
 
-RUN pip3 install -U --no-cache-dir pip && \
-    pip3 install --no-cache-dir -r requirements.txt -r requirements-dev.txt && \
-    pip3 install --no-cache-dir etc/flasgger_package && \
+RUN pip install -U --no-cache-dir pip && \
+    pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt && \
+    pip install --no-cache-dir etc/flasgger_package && \
     make test && \
     python setup.py sdist bdist_wheel --universal