Containers are a technnology for packaging the (compiled) code for an application along with the dependencies it needs at run time. Each container that you run is repeatable; the standardisation from having dependencies included means that you get the same behavior wherever you run it.
Containers decouple applications from underlying host infrastructure. This makes deployment easier in different cloud or OS environments.
A container image is a ready-to-run software package, containing everything needed to run an application: the code and any runtime it requires, application and system libraries, and default values for any essential settings.
By design, a container is immutable: you cannot change the code of a container that is already running. If you have a containerized application and want to make changes, you need to build a new container that includes the change, then recreate the container to start from the updated image.
The container runtime is the software that is responsible for running containers.
Kubernetes supports several container runtimes: DockerDocker is a software technology providing operating-system-level virtualization also known as containers. , containerdA container runtime with an emphasis on simplicity, robustness and portability , CRI-OA lightweight container runtime specifically for Kubernetes , and any implementation of the Kubernetes CRI (Container Runtime Interface).
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