Difference between a Docker image and a container

Understanding the difference between a Docker image and a container is fundamental to working effectively with Docker:

Docker Image

1. Definition:

   • Docker Image: A Docker image is a read-only template containing instructions for creating a Docker container. It includes everything needed to run an application - the code, runtime, libraries, environment variables, and configuration files.

2. Purpose:

   • Images are used to package and distribute applications and their dependencies in a portable format.
   • They serve as a blueprint for creating containers, defining what the container will be and do when it runs.

3. Characteristics:

   • Immutable: Once built, images are immutable and do not change during container runtime.
   • Layered: Images are composed of multiple layers (e.g., base image layer, application layer, dependencies layer) that are stacked on top of each other.
   • Versioned: Images can have different versions (tags) to differentiate between different builds or versions of an application.

4. Example:

   • Pulling an image: docker pull nginx
   • Building an image: docker build -t myapp .
   • Viewing local images: docker images

Docker Container

1. Definition:

   • Docker Container: A Docker container is a runnable instance of a Docker image. It encapsulates the application and its dependencies, along with an execution environment (OS, system libraries), and runs as an isolated process on the host machine.

2. Lifecycle:

   • Containers are created from images using the docker run command.
   • They can be started, stopped, deleted, and their state can be inspected (docker ps, docker inspect).

3. Characteristics:

   • Runnable: Containers are the actual instances of images that run applications in an isolated environment.
   • Lightweight: Containers share the host system's kernel and resources, making them lightweight and efficient.
   • Mutable: Containers can be modified during runtime (e.g., by writing to the filesystem), but changes are lost when the container is removed unless committed to a new image.

4. Example:

   • Running a container: docker run -d --name mynginx nginx
   • Viewing running containers: docker ps
   • Inspecting container details: docker inspect mynginx

Key Differences

• Purpose: Images are used for packaging and distribution, while containers are the runtime instances of those images.

• Mutability: Images are immutable (unchangeable), while containers can be modified during runtime (though changes are not persisted unless committed to a new image).

• Lifecycle: Images persist until deleted, while containers have a lifecycle that includes creation, running, stopping, and deletion.