update the linux build environment to reflect docs.rs

Author: pietroalbini

Diff for: Dockerfile

@@ -1,14 +1,9 @@
-FROM debian:stretch
+FROM ubuntu:bionic
 
 # Install the packages contained in `packages.txt`
 COPY packages.txt /opt/crates-build-env/packages.txt
 RUN apt-get update && cat /opt/crates-build-env/packages.txt | DEBIAN_FRONTEND=noninteractive xargs apt-get install -y
 
-# Install the packages contained in `packages-backports.txt`
-RUN echo "deb http://deb.debian.org/debian stretch-backports main" > /etc/apt/sources.list.d/stretch-backports.list
-COPY packages-backports.txt /opt/crates-build-env/packages-backports.txt
-RUN apt-get update && cat /opt/crates-build-env/packages-backports.txt | DEBIAN_FRONTEND=noninteractive xargs apt-get install -t stretch-backports -y
-
 # Setup the entrypoint
 COPY entrypoint.sh /opt/crates-build-env/entrypoint.sh
 ENTRYPOINT ["/opt/crates-build-env/entrypoint.sh"]

Diff for: ci/lint.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 IFS=$'\n\t'
 
 # Ensure packages lists are sorted
-for file in packages packages-backports; do
+for file in packages; do
     cat "${file}.txt" | sort > "/tmp/sorted-${file}.txt"
     if ! diff "/tmp/sorted-${file}.txt" "${file}.txt" >/dev/null 2>&1; then
         echo "Lint error: ${file}.txt is not sorted"