Use less flaky mirrors

Dockerfile

@@ -1,5 +1,10 @@
 FROM debian:stretch
 
+# The default mirrors are too flaky to run reliably in CI.
+RUN sed -E \
+    '/security\.debian/! s@http://[^/]+/@http://mirrors.kernel.org/@' \
+    -i /etc/apt/sources.list
+
 RUN apt-get update  \
     && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         build-essential \

ci/docker

@@ -8,7 +8,7 @@ chmod 1777 /tmp
 if grep -qE '\<lucid\>' /etc/apt/sources.list; then
     # lucid was archived, breaking the base image
     # https://github.com/docker-library/official-images/issues/1902
-    sed -i 's/archive\.ubuntu\.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
+    sed -i 's@archive\.ubuntu\.com@archive.kernel.org/ubuntu-archive@g' /etc/apt/sources.list
 
     apt-get update
     apt-get install -y --no-install-recommends python-software-properties
@@ -17,6 +17,11 @@ if grep -qE '\<lucid\>' /etc/apt/sources.list; then
     add-apt-repository ppa:fkrull/deadsnakes
     # for zsh 5 backport (lucid's zsh segfaults)
     add-apt-repository ppa:blueyed/ppa
+else
+    # The default mirrors are too flaky to run reliably in CI.
+    sed -E \
+        '/security\.debian/! s@http://[^/]+/@http://mirrors.kernel.org/@' \
+        -i /etc/apt/sources.list
 fi
 
 apt-get update