How to create a copy of exisiting docker image

Question

I've been trying to use docker for a while, these are docker images I've downloaded.

debian                 latest    8c00acfb0175        2 weeks ago         125.2 MB
sameersbn/postgresql   latest    1da0f78d061e        3 weeks ago         231.6 MB
ubuntu                 latest    91e54dfb1179        5 weeks ago         188.4 MB
docker/whalesay        latest    fb434121fc77        4 months ago        247 MB
sameersbn/postgresql   9.4       271ad7e099d3        7 months ago        228.2 MB

How can I've base image for debian or ubuntu locally, and build some testing images on top of them and not disturb base images, so that I don't have to download base docker images again, and I can use fresh downloaded images multiple time ?

Answer 1

Simply write a Dockerfile starting with:

FROM debian:latest
...

(using the FROM directive)

That will create a local image based on debian, and since debian is already downloaded, it won't be downloaded again.

Note: it is best to avoid the "latest" tag: see "Docker: The latest Confusion" by Stack Overflow contributor Adrian Mouat.
Using actual labels is more precise:

docker pull debian:7.8 docker pull debian:wheezy

If wanted to do something in ubuntu is there a way when: I just execute command docker copy "image_name" and then do whatever I want to (run image, clone some git repo, install some packages, test it) , and then just delete it docker rmi "image_name" (when I'm done with image) .

Yes: you can docker run --it <image> bash (fomr images which includes bash), and exit that bash: your container will be exited: you can then docker commit <containerrid> newimage, and you will get a copy of the original image.

Answer 2

The images what you listed using the command "docker images" can be re-used. Here are 2 ways how you can extend or re-use an base image without downloading it again.

1. Using Dockerfile, link

In Dockerfile you can put the Instruction "FROM REPOSITORY_NAME:TAG_NAME" as first line to re-use already downloaded base image

FROM REPOSITORY_NAME:TAG_NAME

In "docker images" command output, 1st column gives REPOSITORY name and 2nd column give TAG name.

You can further add more instructions in Dockerfile and build it.

2. Using Docker commit feature ( not as easy as Dockerfile approach)

a. Start the container from the base image

b. Login to the container, make the changes to the container like install additional rpms etc if required.

c. commit the results for new image created on base image.

docker commit -m "installed wireshark" -a "admin" "container-id" "your_repository_name"/"user_name":"New_TAG"

d. Run "docker images" command, you can see the new image (which is prepared on top of existing, already downloaded image)