I had some personal docker compose'd containers that I wanted to backup without creating a cronjob for each. This backs up all db containers to S3 assuming they're named *_db_1

backup-docker-databases.sh

#!/bin/bash

S3KEY=""
S3SECRET=""

function putS3
{
  path=$1
  file=$2
  aws_path=$3
  bucket='bucket-name'
  date=$(date +"%a, %d %b %Y %T %z")
  acl="x-amz-acl:public-read"
  content_type='application/x-compressed-tar'
  string="PUT\n\n$content_type\n$date\n$acl\n/$bucket$aws_path$file"
  signature=$(echo -en "${string}" | openssl sha1 -hmac "${S3SECRET}" -binary | base64)
  curl -X PUT -T "$path/$file" \
    -H "Host: $bucket.s3.amazonaws.com" \
    -H "Date: $date" \
    -H "Content-Type: $content_type" \
    -H "$acl" \
    -H "Authorization: AWS ${S3KEY}:$signature" \
    "https://$bucket.s3.amazonaws.com$aws_path$file"
}

# Selects the composer started *_db_1 containers
NAME="_db_1"
TIMESTAMP=$(date +"%Y-%m-%d_%H-%M-%S")
BASEDIR=$(dirname "$0")

mkdir -p $BASEDIR/db-backups

for container_name in $(docker ps  --filter="name=$NAME" --format '{{.Names}}')
do
  output="$BASEDIR/db-backups/$container_name.sql"
  docker exec $container_name /usr/bin/mysqldump -uroot --all-databases > $output;
done

TARFILE=db-backup-$TIMESTAMP.tgz
tar -czf $BASEDIR/$TARFILE $BASEDIR/db-backups/
putS3 $BASEDIR $TARFILE "/backups/docker/database/"

# Cleanup
rm -rf $BASEDIR/db-backups $BASEDIR/*.tgz