Upgrade Buildserver VM

Use Vagrant boxes built with cloud-team/debian-vagrant-images instead of fdroid/basebox,
Use Debian Bullseye (11) instead of Debian Stretch (9)

buildserver/Dockerfile

@@ -1,5 +1,5 @@
 
-FROM debian:stretch
+FROM debian:bullseye
 
 ENV LANG=C.UTF-8 \
     DEBIAN_FRONTEND=noninteractive

buildserver/provision-apt-get-install

@@ -37,14 +37,11 @@ if echo $debian_mirror | grep '^https' 2>&1 > /dev/null; then
 fi
 
 cat << EOF > /etc/apt/sources.list
-deb ${debian_mirror} stretch main
-deb http://security.debian.org/debian-security stretch/updates main
-deb ${debian_mirror} stretch-updates main
+deb ${debian_mirror} bullseye main
+deb http://security.debian.org/debian-security bullseye-security main
+deb ${debian_mirror} bullseye-updates main
 EOF
-echo "deb ${debian_mirror} stretch-backports main" > /etc/apt/sources.list.d/stretch-backports.list
-echo "deb ${debian_mirror} stretch-backports-sloppy main" > /etc/apt/sources.list.d/stretch-backports-sloppy.list
-echo "deb ${debian_mirror} testing main"           > /etc/apt/sources.list.d/testing.list
-printf "Package: *\nPin: release o=Debian,a=testing\nPin-Priority: -300\n" > /etc/apt/preferences.d/debian-testing
+echo "deb ${debian_mirror} bullseye-backports main" > /etc/apt/sources.list.d/backports.list
 
 dpkg --add-architecture i386
 
@@ -56,7 +53,7 @@ apt-get upgrade
 apt-get update || apt-get update
 
 packages="
- androguard/stretch-backports
+ androguard
  ant
  asn1c
  ant-contrib
@@ -80,10 +77,10 @@ packages="
  git-core
  git-svn
  gperf
- gpg/stretch-backports-sloppy
- gpgconf/stretch-backports-sloppy
- libassuan0/stretch-backports
- libgpg-error0/stretch-backports
+ gpg
+ gpgconf
+ libassuan0
+ libgpg-error0
  javacc
  libarchive-zip-perl
  libexpat1-dev
@@ -110,7 +107,7 @@ packages="
  python-magic
  python-pip
  python-setuptools
- python3-asn1crypto/stretch-backports
+ python3-asn1crypto
  python3-defusedxml
  python3-git
  python3-gitdb