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Build Your Own PaaS with Docker by Oskar Hane

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The Docker container

A Docker container is created the moment we execute docker run imagename. A writeable layer is added on top of all the image layers. This layer has processes running on the CPU and can have two different states: running or exited. This is the container. When we start a container with the Docker run command, it enters the running state until it, for some reason, stops by itself or is stopped by us and then enters the exited state.

When we have a container running, all the changes we make to its filesystem are permanent between start and stop. Remember that changes made to the container's filesystem are not written to the underlying Docker image.

We can start as many instances of running containers as we want from the same image; ...

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