Dockerfile: installing future before requirements.txt

Dockerfile

@@ -17,6 +17,7 @@ WORKDIR /freqtrade
 # Install dependencies
 COPY requirements.txt /freqtrade/
 RUN pip install numpy --no-cache-dir \
+  && pip install future \
   && pip install -r requirements.txt --no-cache-dir
 
 # Install and execute