Add a script to run backups every 15 minutes.

Right now, the script just does a database dump, of the `mysql` and
Bookstack databases.  Each backup is written into the Bookstack data
volume, at the `/backups` path.

TODO: Kick off Restic backups.

root/etc/periodic/15min/backup.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# vim: ts=4 sw=4 noet
+
+# This script runs our backups!
+
+set -eu
+set -o pipefail
+
+# PART 0: Check environment
+if [ ${MYSQL_HOST:-x} = 'x' ]; then
+	echo 'MYSQL_HOST environment variable is missing!'
+	exit 1
+fi
+if [ ${DB_PASS_FILE:-x} = 'x' ]; then
+	echo 'DB_PASS_FILE environment variable is missing!'
+	exit 1
+fi
+if [ ${DB_DATABASE:-x} = 'x' ]; then
+	echo 'DB_DATABASE environment variable is missing!'
+	exit 1
+fi
+
+# PART 1: Dump the database into the Bookstack directory
+mysqldump --host=${MYSQL_HOST} --password=$(cat $DB_PASS_FILE) --databases mysql > /bookstack/backups/mysql.sql
+mysqldump --host=${MYSQL_HOST} --password=$(cat $DB_PASS_FILE) --databases ${DB_DATABASE} > /bookstack/backups/bookstack.sql
+
+# PART 2: Run a Restic backup of the Bookstack directory