I want to put a docker image inside docker Repo or want to create an image on local machine . but I am unable to understand what I should put in the Image and how to create it.

I want to create a Docker image which contains Java and PostgreSQL. I just want to create an Image to reuse it from anywhere.

From reading the documentation I don't understand how I can do that.

This is what I tried:

user@host:/$ docker run -i -t debian /bin/bash 
root@container:/$ apt-get install postgresql-9.3
user@host:/$ docker ps 
user@host:/$ docker commit <CID> username/postgresql

I am new to Docker technology , I installed boot2Docker & oracle's VM on my windows and tried some examples which are picking examples from Github. Now i want to put a docker image inside docker Repo or want to create an image on local machine . but I am unable to understand what I should put in the Image and how to create it.

I want to put a docker image inside docker Repo or want to create an image on local machine . but I am unable to understand what I should put in the Image and how to create it.