Update Dockerfile (#1451)

- Forget one "\" in the process!

Dockerfile

@@ -32,7 +32,7 @@ RUN echo "@commuedge http://nl.alpinelinux.org/alpine/edge/community" >> /etc/ap
     openssl-dev \
     ca-certificates \
     tini@commuedge \
- && pip install --upgrade pip   
+ && pip install --upgrade pip \
  && pip install --no-cache -r requirements.txt \
  && apk del \
     build-base \