For `Flink/dokku-psql-single-container`. See https://gist.github.com/henrik/26bb73091712aa42abf2#db-backups

dokku_postgres_backups.sh

#! /bin/bash
# For use with Flink/dokku-psql-single-container.
# Based on http://donpottinger.net/blog/2014/11/25/postgres-backups-with-dokku.html

set -e

BASE_DIR="/var/backups/postgres"
mkdir -p $BASE_DIR

YMD=$(date "+%Y-%m-%d")
FILE="$BASE_DIR/$YMD.dump"

/usr/bin/docker exec psql-single-container su postgres -c "pg_dumpall" > "$FILE"

# delete backup files older than 10 days
# (since Digital Ocean disk backups can be as infrequent as every 7 days)
OLD=$(find $BASE_DIR -mtime +10)
if [ -n "$OLD" ] ; then
  echo deleting old backup files: $OLD
  echo $OLD | xargs rm -rfv
fi

Also see Flink/dokku-psql-single-container#17 (comment) which gets all DBs but as separate dumps.