Trac #29091: tox.ini: Add raspbian-buster

On Raspberry Pi 4 4GB on Raspbian Buster build failed with fplll:
{{{
libtool: link: g++ -std=gnu++11 -std=c++11 -fPIC
-I/home/pi/sagemath/sage-9.1.be
ta1/local/include/ -O3 -Wl,-rpath
-Wl,/home/pi/sagemath/sage-9.1.beta1/local/lib
 -o .libs/fplll main.o  -L/home/pi/sagemath/sage-9.1.beta1/local/lib
-L/lib ./.l
ibs/libfplll.so -lmpfr -lgmp -pthread -Wl,-rpath
-Wl,/home/pi/sagemath/sage-9.1.
beta1/local/lib
/usr/bin/ld: ./.libs/libfplll.so: undefined reference to
`__atomic_store_8'
/usr/bin/ld: ./.libs/libfplll.so: undefined reference to
`__atomic_load_8'
collect2: error: ld returned 1 exit status
}}}

This is fixed by upgrading FPLLL in #30021.

In this ticket, we add Docker-based testing for the platform `raspbian-
buster-armhf` so that it can be verified that FPLLL now builds on this
platform by the following:
{{{
  tox -e docker-raspbian-buster-armhf-standard -- fplll
}}}

URL: https://trac.sagemath.org/29091
Reported by: jsp
Ticket author(s): Matthias Koeppe
Reviewer(s): Jaap Spies

build/bin/write-dockerfile.sh

@@ -43,6 +43,11 @@ case $SYSTEM in
 ARG BASE_IMAGE=ubuntu:latest
 FROM \${BASE_IMAGE} as with-system-packages
 EOF
+        if [ -n "$DIST_UPGRADE" ]; then
+            cat <<EOF
+RUN sed -i.bak $DIST_UPGRADE /etc/apt/sources.list && apt-get update && DEBIAN_FRONTEND=noninteractive apt-get -y dist-upgrade
+EOF
+        fi
         EXISTS="2>/dev/null >/dev/null apt-cache show"
         UPDATE="apt-get update &&"
         INSTALL="DEBIAN_FRONTEND=noninteractive apt-get install -qqq --no-install-recommends --yes"

tox.ini

@@ -249,8 +249,13 @@ setenv =
     #
     raspbian:         SYSTEM=debian
     raspbian:         ARCH_IMAGE_PREFIX=
+    raspbian:                                  IGNORE_MISSING_SYSTEM_PACKAGES=yes
     raspbian-jessie:  BASE_IMAGE=raspbian/jessie
     raspbian-stretch: BASE_IMAGE=raspbian/stretch
+    # As of 2020-06, there is no raspbian/buster image, so we dist-upgrade from stretch.
+    # https://www.raspberrypi.org/blog/buster-the-new-version-of-raspbian/
+    raspbian-buster:  BASE_IMAGE=raspbian/stretch
+    raspbian-buster:      DIST_UPGRADE=s/stretch/buster/g
     #
     # https://hub.docker.com/u/linuxmintd
     #