Remove automatically installed packages whenever possible

This marks all packages installed by `debootstrap` as automatic and
sets up policy for pruning purposes before purging unneeded ones.
The devuan-keyring is explicitly kept.

bootstrap.sh

@@ -17,9 +17,15 @@ debootstrap \
     --no-merged-usr \
     --variant=minbase jessie $rootfs
 
+chroot $rootfs apt-mark auto '.*' \
+       | sed '/not installed/d'
+chroot $rootfs apt-mark manual devuan-keyring
+
 bindir=usr/local/bin
 install -m 0755 -o root -g root scripts/* $rootfs/$bindir
 
 for script in $(ls $rootfs/$bindir); do
     chroot $rootfs /$bindir/$script
 done
+
+chroot $rootfs apt-get --purge autoremove -q -y

scripts/tweak-apt-config

@@ -27,6 +27,12 @@ Acquire::CompressionTypes::Order { "gz"; };
 EOF
 chmod 0644 $cfg/docker-compress
 
+cat > $cfg/docker-suggests <<- EOF
+APT::Install-Suggests "false";
+APT::AutoRemove::SuggestsImportant "false";
+EOF
+chmod 0644 $cfg/docker-suggests
+
 cat > $cfg/docker-translations <<- EOF
 Acquire::Languages "none";
 EOF