Update Debian repos

Dockerfile

@@ -25,8 +25,8 @@ RUN printf '#-------------------------------------------------------------------
 \ndeb-src http://deb.debian.org/debian-security	'${DEB_RELEASE}'/updates		main # contrib non-free\
 \n\
 \n# stretch-backports, previously on backports.debian.org\
-\ndeb     http://ftp.debian.org/debian		'${DEB_RELEASE}'-backports	main # contrib non-free\
-\ndeb-src http://ftp.debian.org/debian		'${DEB_RELEASE}'-backports	main # contrib non-free\
+\ndeb     http://deb.debian.org/debian		'${DEB_RELEASE}'-backports	main # contrib non-free\
+\ndeb-src http://deb.debian.org/debian		'${DEB_RELEASE}'-backports	main # contrib non-free\
 \n' > /etc/apt/sources.list
 
 # Build deps and clean up