dokku postgres backup cronjob (using https://github.com/Kloadut/dokku-pg-plugin)

dokku-pg-backup.sh

#! /bin/bash

# directory to save backups in, must be rwx by postgres user
BASE_DIR="/var/backups/postgres"
YMD=$(date "+%Y-%m-%d")
DIR="$BASE_DIR/$YMD"
mkdir -p $DIR
cd $DIR

# make database backup
dokku postgresql:dump $1 | gzip -9 > "$DIR/db.out.gz"

# delete backup files older than 7 days
OLD=$(find $BASE_DIR -type d -mtime +7)
if [ -n "$OLD" ] ; then
        echo deleting old backup files: $OLD
        echo $OLD | xargs rm -rfv
fi

On Ubuntu 16.04.03 with the last version of dokku and postresql plugin.
I change this line on the script to make it works :

dokku postgresql:dump $1 > "$DIR/db.dump"
TO
dokku postgres:export $1 > "$DIR/db.dump"

Also :
Dont forget to make it executable with sudo chmod +x yourscript.sh.
If you want to test it, do it with sudo ./yourscript.sh DBName
If you want to create a cron, use sudo crontab -e