Properly handle dokku data removal

- remove deployed applications during dpkg purge
- disable all core plugins during dpkg purge
- ensure init files are removed
- ensure motd is removed
- remove all core plugins, dokku config files (including tls/sshcommand entries etc.) during purge
- remove all empty directories and broken symlinks in the DOKKU_ROOT and DOKKU_LIB_ROOT directories during purge

Note that we do not delete the dokku user itself as there might be other data we don't know about in this directory.

debian/postrm

@@ -1,7 +1,39 @@
 #!/bin/bash
 set -eo pipefail; [[ $TRACE ]] && set -x
 
-if [[ "$1" == "purge" ]] && [[ -e /usr/share/debconf/confmodule ]]; then
-    . /usr/share/debconf/confmodule
-    db_purge
-fi
+readonly DOKKU_ROOT="${DOKKU_ROOT:-/home/dokku}"
+readonly DOKKU_LIB_ROOT="${DOKKU_LIB_PATH:-/var/lib/dokku}"
+
+main() {
+  rm -f /etc/init/dokku-installer.conf
+  rm -f /etc/init/dokku-redeploy.conf
+  rm -f /etc/systemd/system/dokku-installer.conf
+  rm -f /etc/systemd/system/dokku-redeploy.service
+  rm -f /etc/update-motd.d/99-dokku
+
+  if [[ "$1" == "purge" ]]; then
+    rm -f ${DOKKU_ROOT}/.dokkurc ${DOKKU_ROOT}/dokkurc ${DOKKU_ROOT}/tls
+    rm -f ${DOKKU_ROOT}/.ssh/authorized_keys ${DOKKU_ROOT}/.sshcommand
+    rm -f ${DOKKU_ROOT}/ENV ${DOKKU_ROOT}/HOSTNAME ${DOKKU_ROOT}/VERSION
+    rm -rf ${DOKKU_ROOT}/.cache
+    rm -rf ${DOKKU_LIB_ROOT}/core-plugins
+
+    rm -f ${DOKKU_LIB_ROOT}/plugins/config.toml
+    if [[ -d ${DOKKU_LIB_ROOT} ]]; then
+      find -L ${DOKKU_LIB_ROOT} -type l -delete
+      find ${DOKKU_LIB_ROOT} -type d -empty -delete
+    fi
+
+    if [[ -d ${DOKKU_ROOT} ]]; then
+      find -L ${DOKKU_ROOT} -type l -delete
+      find ${DOKKU_ROOT} -type d -empty -delete
+    fi
+
+    if [[ -e /usr/share/debconf/confmodule ]]; then
+      . /usr/share/debconf/confmodule
+      db_purge
+    fi
+  fi
+}
+
+main "$@"