Docker image vs container: Key differences explained

A Docker container is a virtual environment where you can run an application, along with its dependencies and other components, in complete isolation from other apps. Meanwhile, a Docker image is a file containing the instructions for building a container, defining aspects like its configuration and the software to install.

Docker containers and images also differ in several other aspects. Here’s a quick comparison of how they fare in terms of mutability, lifecycle, storage, portability, commands, and usage:

Differentiator	Containers	Images
Mutability	Mutable, editable at runtime.	Immutable, only customizable as a Dockerfile before the build.
Lifecycle	Go through states such as creation, running, paused/unpaused, stopped, and deleted. Often recreated throughout the development process.	Go through the creation, versioning, and pruning phases. Typically stays longer at rest.
Storage	Only on the production server or the development machine.	Multiple locations, including the production server, local development machine, public registry, and private repository.
Portability	Can’t be moved between systems, only transportable as an image.	Portable and can be easily moved between systems.
Commands	docker run, docker ps, docker exec, docker stop, docker start, docker rm, docker logs	docker build, docker pull, docker push, docker images, docker rmi, docker tag
Usage during development	Setting up a testing environment that is isolated and independent from other services.	Maintaining a consistent testing environment across multiple systems during development.
Usage during production	Deploying scalable applications as smaller services or on multiple servers.	Packaging applications with their dependencies and settings for streamlined deployment.

Let’s explore the key differences between Docker images and containers in detail, comparing their mutability, storage, portability, commands, and usage in software deployment.

Docker container overview

A container in Docker is an isolated virtual environment that runs independently. It includes all components required to run an application, including dependencies, libraries, and code.

Think of a Docker container as an apartment unit within a building. It contains all the necessary facilities for the tenant to live independently of their neighbors.

For example, an online store application might consist of several services, such as the front end, back end, and database. These components all use different tools, programs, and code.

You can pack each service in a container with its required components. This enables the services to run independently despite being hosted on the same server, allowing you to easily manage and modify them.

Docker image overview

A Docker image contains instructions on how to build a container, including the required components and source code. It lets you set up a consistent environment easily and maintain compatibility across multiple systems.

If Docker containers are apartment units within a building, Docker images serve as the floor plan. It specifies the components required to build the room and the furniture needed for the tenant to live comfortably.

You can build multiple containers from the same image, each with identical application, components, and configuration.

Docker image vs container compared

After exploring the general definition of Docker images and containers, let’s see how they differ in several key aspects.

Which is more mutable: Docker images or containers?

Docker images are read-only once built, meaning users can’t change the configuration to build a different container. For example, you can’t add a new software package or edit already-specified dependencies.

To modify the configuration, you must make a new Docker image from a Dockerfile, a human-readable text in which you write the necessary components and instructions to create containers.

Meanwhile, a Docker container is editable at runtime. You can change the application code, add a new program, or edit configuration files freely without affecting another container.

Which has a more flexible lifecycle: Docker image or container?

Docker images and containers go through different phases during the application deployment process. However, the lifecycle phases can differ depending on your project’s management policy.

A Docker image’s lifecycle typically consists of the following stages:

• Creation – the creation stage is where you build a Docker image instance from a Dockerfile.
• Tagging and versioning – developers label each Docker image to determine which container it builds and its iteration.
• Pruning – developers will delete old Docker images and replace them with new ones from a fresh Dockerfile.

Meanwhile, a Docker container’s lifecycle might appear as follows:

• Creation – the developers build the container from a Docker image.
• Running – the container starts and actively executes its process.
• Paused/unpaused – the administrators halt or resume the container operation.
• Stopped – the container stops after completing its process, and you can restart it later.
• Deleted – the container no longer exists, meaning you must recreate it from the image.

Images typically remain in storage long after creation for version control and reuse. However, you might eventually remove Docker images to free up storage space or clean up your environment.

Meanwhile, containers tend to have a shorter lifecycle as developers often recreate, stop, and delete them throughout the application development process.

Where are Docker images and containers stored?

Docker images can be stored in multiple locations depending on their purpose. For example, you can store them on your local machine or the host server during development. Meanwhile, they reside in a centralized Docker registry at rest, which enables users to save images and share them with other developers.

You can use the official Docker Hub registry, employ third-party services from cloud providers, or self-host the store on a virtual private server platform like the service available from Hostinger.

A private registry is especially helpful when working in a team environment because it provides a higher level of data security and access control. A self-managed repository also enables custom labelling and promotes more consistent versioning, which helps improve development efficiency because you can identify images more easily.

Meanwhile, Docker containers can only be stored on the host system. To share and archive them in different locations, you must use their images.

Which is more portable: Docker image or container?

A Docker image’s portability means you can easily store, share, and reuse it on different systems. Meanwhile, containers can’t be transported by default and must be moved as images.

You can move images and build containers on any system that supports Docker. However, due to configuration differences, containers might behave differently on another machine.

To minimize compatibility issues when running containers on different servers, Docker provides the docker-compose utility that enables you to orchestrate images, and therefore containers, more consistently on multiple environments.

Which commands are used for Docker images vs containers?

Commands for interacting with images and containers are different. For instance, here are the utilities for Docker images:

• docker build – creates a Docker image from a Dockerfile.
• docker pull – downloads an image from a Docker registry.
• docker push – uploads a local image to a registry.
• docker images – lists all locally available images.
• docker rmi – removes an image from the local system.
• docker tag – adds a new tag to an image for versioning.

Meanwhile, Docker container commands are as follows:

• docker run – creates and starts a container from an image.
• docker ps – lists all running containers.
• docker exec – runs a command inside a running container.
• docker stop – stops a running container.
• docker start – starts a stopped container.
• docker rm – removes a stopped container.
• docker logs – prints logs from a running or stopped container.

How are Docker images and containers used in development and production?

Docker images enable you to set up and replicate the testing area on different machines during development. It is especially helpful in a team environment since members don’t need to install each component manually.

Images’ portability and consistency also help streamline deployment. Since you can pack your application and its components into a single image, you can quickly push your project into production environments without having to prepare the dependencies.

Meanwhile, Docker containers in development provide an isolated environment where developers can freely test changes. It prevents modifications from affecting other services and minimizes the risk of conflicts between components.

Docker containers in production allow you to host your application using multiple redundant nodes, which optimize resource utilization and service availability. You can also deploy your project as a microservice and avoid vendor lock-in by distributing containers on different systems.

When to use Docker images and containers?

Both Docker images and containers are used equally in real-world scenarios because they work hand-in-hand. However, you typically utilize them at different stages of your development.

Docker images are crucial at the initial stage of development because you will use them to pack your application and its components. You will also use them to set up the environment on different machines and track development iteration.

Meanwhile, Docker containers are useful in the production phase, during which your application is already running. You can also use them to scale your project up or down by managing each environment.

We recommend checking our tutorial on creating a Docker container to learn more about the development sequence in real-world applications.

Next Steps with Docker: Expanding your knowledge

Docker containers are mutable environments used to deploy services in a host system. You can start, stop, recreate, and delete them according to your project lifecycle. Meanwhile, Docker images are immutable but portable blueprints defining how to create containers.

Containers and images are only two of many essential components of Docker’s containerization technology. If you are looking to utilize Docker for your project, we highly recommend learning more about this tool by deploying it yourself and exploring its capabilities hands-on.

For beginners, Hostinger can be an excellent option. In addition to easy-to-use VPS plans, we provide a wide range of guides about Docker to help you learn about the tool more easily. From installing Docker on your server to using advanced features like container orchestration, we have tutorials explaining how to do it.

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