postgresql - Docker Backup and restore postgres -

i using docker postgres:9.4 image. need know how backup , restore volume container.

created volume container:

docker run -v /var/lib/postgresql/data --name dbdata postgres:9.4 /bin/true 

using volume

docker run --name=postgres --volumes-from=dbdata -d -p 6432:5432 postgres:9.4 

backup volume container

docker run --volumes-from dbdata -v $(pwd):/backup postgres:9.4 tar cvf /backup/backup.tar /var/lib/postgresql/data 

restore volume in new container

docker run --name=dbdata-new --volumes-from dbdata -v $(pwd):/backup ubuntu:14.04 /bin/sh -c 'cd /var/lib/postgresql/data && tar xvf /backup/backup.tar' 

use in new volume in creating new postgres container:

docker run --name=postgres-new --volumes-from=dbdata-new -d -p 7532:5432 postgres:9.4 

issue: i below error in logs when run new container.

initdb: directory "/var/lib/postgresql/data" exists not empty if want create new database system, either remove or empty directory "/var/lib/postgresql/data" or run initdb argument other "/var/lib/postgresql/data"

not sure doing wrong. can please point out making mistake.

could not reproduce issue following steps rudiment data (one record 1 table 1 new db):

psql -u postgres -h $(boot2docker ip || echo 'localhost') -p 6432 -c "create database ttt;" psql -u postgres -h $(boot2docker ip || echo 'localhost') -p 6432 -d ttt -c "create table a(b int); insert a(b) values(1);" psql -u postgres -h $(boot2docker ip || echo 'localhost') -p 6432 -d ttt -c "select * a;" 

when start postgres-new no exceptions in logs , data seems there:

$ psql -u postgres -h $(boot2docker ip || echo 'localhost') -p 7532 -d ttt -c "select * a;"  b ---  1 (1 row)