Make dockerfile valid

Dockerfile

@@ -1,6 +1,7 @@
 FROM debian:stretch-slim
+RUN echo 'deb http://ftp.de.debian.org/debian buster-backports main' >> /etc/apt/sources.list
 RUN DEBIAN_FRONTEND=noninteractive DEBIAN_PRIORITY=critical apt-get -qqy --no-install-recommends -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confold -o Dpkg::Options::=--force-unsafe-io update \
- && DEBIAN_FRONTEND=noninteractive DEBIAN_PRIORITY=critical apt-get -qqy --no-install-recommends -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confold -o Dpkg::Options::=--force-unsafe-io install python python2.7 rsync git ca-certificates curl
+ && DEBIAN_FRONTEND=noninteractive DEBIAN_PRIORITY=critical apt-get -qqy --no-install-recommends -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confold -o Dpkg::Options::=--force-unsafe-io install python python2.7 rsync git ca-certificates curl python-pip \
  && DEBIAN_FRONTEND=noninteractive DEBIAN_PRIORITY=critical apt-get -qqy --no-install-recommends -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confold -o Dpkg::Options::=--force-unsafe-io -t buster-backports install zstd libzstd-dev libzstd1 \
  && pip install zstandard
 COPY * factory/