Improve and document backup script, add scp option (#429)

* Improve and document backup script, add scp option

* dump_db as helper function

* Add option to only dump the database

helpers.sh

@@ -145,4 +145,16 @@ btcpay_restart() {
         docker-compose -f $BTCPAY_DOCKER_COMPOSE restart
     fi
     popd > /dev/null
-}
+}
+
+btcpay_dump_db() {
+    pushd . > /dev/null
+    cd "$(dirname "$BTCPAY_ENV_FILE")"
+    backup_dir="/var/lib/docker/volumes/backup_datadir/_data"
+    if [ ! -d "$backup_dir" ]; then
+        docker volume create backup_datadir
+    fi
+    local filename=${1:-"postgres-$(date "+%Y%m%d-%H%M%S").sql"}
+    docker exec $(docker ps -a -q -f "name=postgres_1") pg_dumpall -c -U postgres > "$backup_dir/$filename"
+    popd > /dev/null
+}