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Docker Image vs Container: The Key Differences

05th Sep, 2023
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    Docker Image vs Container: The Key Differences

    It is always wise to first understand the concept of subjects that are under consideration in our discussion here. In our case, we are trying to decode the difference between the Docker image and container, which we know are the core Objects of the Docker as an ecosystem. Many people also call this comparison as Docker Vs Containers which I feel is not right to do because one needs to first understand the core objects which make docker system and try to decode those objects if they want to compare them  

    So let’s first define 

    • What Is Docker Image? 
    • What Is Docker Container?  

    What is Docker Image

    Let’s start by understanding - 

    What Is Docker? 

    Well, I always like to call Docker a mammoth cargo ship that has the ability to hold big boxes (containers) having their distinct objectives and id. These boxes contain items that are unique and are required to make that particular box useful for the company/person who has asked for it to be shipped. These items are first manufactured in factories that have templates to reproduce such items. These items (images) are packaged into the boxes (dockerized/containerized) in such a manner that they are useful for someone who has asked them to be shipped.  

    If you are finding this analogy hard to digest, let me define it by keeping its technical aesthetics in place 

    Docker is open-source, virtualization software created to make developer's life easy. It is a kind of PaaS (platform-as-a-service) product whose core objective is to isolate virtual environments to deploy, build, and test applications that are usually incompatible or not meant to work with the current OS. 

    So let’s come back to docker images.  

    Docker Images

    It is a kind of ready-to-use software read-only template crafted with source codes, libraries, external dependencies, tools, and other miscellaneous files that are needed for any software application to run successfully on any platform or OS.  

    The developer community also likes to call it Snapshots, representing the app and its virtual environment at a specific point in time.  

    This docker snapshot is a perfect recipe for developers to build test and deploy the desired app swiftly. Know more about introduction to docker, docker containers & docker hub

    What is Docker Container?

    Now that you have got hold of the docker image concept it will be apt now to describe the docker container.  

    As we discussed, docker images are nothing but a read-only template that can’t be executed by themselves and cannot run or start. If that is the limitation how can one make a real value out of it?  

    Well, the answer lies in the concept of the Docker container.  

    Docker Container 

    Docker Container

    A container is nothing but a box that has the ability to run the docker image templates. The moment you create a container using those immutable images you essentially end up creating a read-write copy of that filesystem (docker image) inside the given container. This adds a container layer which helps you to modify the entire copy of the given Docker image. 

    A container can also be considered as a cohesive software unit that packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another.  

    If your curiosity is not settling down why don’t you go ahead and check out this course learning docker online

    Why Docker Containers Are Useful? 

    All the docker images become docker containers when they run the on Docker Engine and these containers are popular amongst developers and organizations of all shapes and sizes because it is-  

    • Standardized: Docker created the industry standard for containers, so they could be portable anywhere. 
    • Lightweight: Containers share the machine’s OS system kernel and therefore do not require an OS per application, driving higher server efficiencies and reducing server and licensing costs. 
    • Secure: Applications are safer in containers and Docker provides the strongest default isolation capabilities in the industry. 

    Now that we have covered the fundamentals of docker images and containers, we have got the right set of foundations to compare them wisely. Know more about top features of Docker

    How do Docker Images Differ from Containers?

    To be true if you see the docker ecosystem in its entirety you will agree that both of these docker objects are an inclusive part of it. Without docker images, you can imagine docker containers at all and without docker containers, docker images are like completely meaningless and orphans.  

    So is it wise to compare them, I may say no but still to clarify the concepts we can do so only on the basis of their roles and responsibilities  

    Docker Image vs Containers

    • The key difference between a Docker image Vs a container is that a Docker image is a read-only immutable template that defines how a container will be realized. A Docker container is a runtime instance of a Docker image that gets created when the $ docker run command is implemented.  
    • Before the docker container can even exist docker templates/images are built using $ docker build CLI. 
    • Docker image templates can exist in isolation but containers can't exist without images.  
    • So docker image is an integral part of containers that differs only because of their objectives which we have already covered.  
    • Docker images can’t be paused or started but a Docker container is a run time instance that can be started or paused. 

    The docker/docker desktop is one of the core developer tools for startups, growing organizations, or largescale enterprises especially when they are looking to adopt a competitive DevOps culture and ecosystem. So if you want to get a holistic feel of the scalable architecture you should check best DevOps courses online.

    Food for Thought! 

    When it comes to making a choice of tools and tech, especially for start-ups and growing organizations, one should always consider a few pointers which I am listing down  

    • Assess the tech debt in terms of time and cost that may pop up by choosing the one which is aligned to the company's growth and business needs.  
    • Ease of adoption: See how difficult or easy it will be to adopt the new tool for the developers and what is the cost attached to the same 
    • Assess if it is worth taking the pain, when I say this, I mean, see the current and future implications of the change. If the change will, help you scale your tech infra to become more robust and reliable and that can be done without much pain and cost, you can go ahead.  

    I always feel:  

    “No change is bad until you are willing to accept is pros and cons both, with open mind and heart ” 

    Frequently Asked Questions (FAQs)

    1What is the difference between Docker and container?

    It will not be wise to say that docker is different from containers because docker is a complete ecosystem that has two key objects, Docker Image and Docker Containers which can be compared based on their roles and objectives already been covered in detail. 

    2Is Docker a container?

    Not Exactly, docker is container-based technology and containers are just user space of the operating system. At the low level, a container is just a set of processes that are isolated from the rest of the system, running from a distinct docker image that provides all files necessary to support the processes. As discussed docker container is a runnable instance of an image.

    3Is Docker a container or VM?

    Docker is a containerization tech that supports virtualization technology but not the VM itself. Docker containers are considered apt able to run multiple applications over a single OS kernel; whereas, virtual machines are needed if the applications or services required to run on different OS. Docker container packages are self-contained and can run applications in any environment, and since they don’t need a guest OS, they can be easily ported across different platforms.  

    VM differs also in the sense of performance and scalability. VM resources like CPU, memory, and I/O may not be allocated permanently to containers which makes it difficult to scale dynamically. In containers scaling up and replicating the containers is not a problem and that makes it stand out.  

    4Is Docker the only container?

    Being the most loved containerization solution docker does stand out but Docker is not the only container out there. So let me share a few of the Docker alternatives  

    • LXC: a set of low-level container management tools that are part of the open-source project. The technology was a forerunner to Docker and is sponsored by Canonical, the firm behind Ubuntu. 
    • rkt : rkt (formerly known as CoreOS Rocket) has arguably become one of the most viable alternatives to Docker. It is secure, interoperable with other systems and frameworks. 

    To learn more about Kubernetes do check out this course: the best way to learn Docker and Kubernetes

    • Podman: is another open-source container engine, it has got features like isolation and user privilege features that make Podman inherently more secure.  
    5Is Docker a cloud technology?

    Docker is apt for highly portable workloads. Docker containers can run on a developer’s local laptop, on physical or virtual machines in a data center, on cloud providers, or in a mixture of environments. 


    Pramod Chandrayan


    A techie, a marketeer, a storyteller, and an entrepreneur by choice who is currently on a mission to make everyone financially aware and healthy. Currently, he is Co-founder and Chief of Product & Marketing at FinMapp a fintech startup based out of Gurgaon, and mostly spends his time reading and writing about tech, startups, Crypto & Life as a whole.

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