Docker Containers and Images : Part 1

I started learning docker few days back and got confused when it came to differentiating docker containers and images. In a simple definition docker image is manifest or snapshot of container and a container is instance of docker image.

Lets dig into images a bit, We all need a Dockerfile to build an image and we all know that Dockerfile contains commands like FROM, WORKDIR, CMD, RUN, EXPOSE etc. And we all have seen docker creates layer for each command written in Dockerfile on top of last command, so far so good!

Lets create a simple docker file

And execute docker image build -t test001 .

So all commands are getting executed step wise. but wait, what is this Running in 788788020b27 after step 3 and there is another line Removing intermediate container 7887888020b27. hmm.. seems like there is a container executing on top of each layer and its result is getting committed as a snapshot for next step.

So I executed my next command

docker image build -t test001 --rm=false .

Now I do not see any Removing intermediate container and executing docker container ls -a command gives below result

All the containers for each layer is present as stopped container. Now what docker container run do? It creates another R/W layer on top of top layer of image.

So the major difference between image and container is this Thin R/W layer.