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A Docker image is a read-only template; a Docker container is a runnable instance of that image. Docker builds images from layered, read-only filesystems. When it creates a container, it adds a separate writable layer on top. Changes made in that layer belong to that container, not to the image, and disappear with the container unless the data is stored in a volume.
Docker image vs. container at a glance
| Question | Docker image | Docker container |
|---|---|---|
| What is it? | A packaged, read-only filesystem and application template. | A runtime instance made from an image. |
| Can it be changed directly? | No. Its build layers remain read-only. | Yes. Docker gives each container a writable layer. |
| How is it created? | Dockerfile instructions produce its layers. | Docker creates it from an image. |
| Where do ordinary file changes go? | Nowhere; the image itself is unchanged. | Into the container’s writable layer. |
| What happens when it is deleted? | The image remains until you remove it separately. | Its writable layer is removed with the container. |
| Can several use the same base? | Yes. One image can back many containers. | Each container has its own writable state. |
| Is its writable data durable? | Images are not the runtime location for changing application data. | Data in the writable layer is temporary; use a volume for data that must outlive the container. |
What a Docker image is
An image is the packaged starting point for a container. It contains the files, libraries, application code and configuration that Docker needs to create a filesystem for a process. The important property is immutability: an image is assembled from read-only layers, so running an application does not edit the artifact from which it came.
Dockerfile instructions become layers
A Dockerfile describes how an image is assembled. Each instruction contributes a layer. Those layers are stacked in order to form the image that Docker can later use as a container’s base. The layering model allows Docker to retain and reuse unchanged portions instead of treating every build as one monolithic file.
Every image layer is read-only. “Read-only” does not mean an image is unusable; it means that a process cannot persist an ordinary edit back into that layer while a container is running. To change the packaged application, you build a new image (often reusing unchanged layers) rather than editing the old image in place.
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An image can exist without a container
Images are independent artifacts. You can have an image stored locally even when no container currently exists for it. A container, in contrast, requires an image as its initial filesystem and application definition. This is why image management and container management are separate tasks: retaining an image does not keep any particular container’s runtime changes, and deleting a container does not delete the image that produced it.
Images are the shareable build artifact
In a normal Docker workflow, the image is the unit that is built, tagged, stored and transferred between environments. Teams can use the same image as the starting point for development, testing and production. The container is a particular runtime instance, with its own name, status and writable state, rather than the artifact you distribute as the application package.
What a Docker container is
A container is an instance created from an image. At creation time, Docker places a writable “container layer” above the image’s read-only layers. The process running in the container sees a unified filesystem, but Docker keeps the immutable base and the instance-specific changes conceptually separate.
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When a process creates, edits or deletes a file in a container, Docker records that operation in the container’s writable layer. The original file in an image layer is not rewritten. If a container changes a configuration file that came from the image, the container gets its own changed view; another container created from the same image continues to see the original file until it makes a change of its own.
Stopping and deleting are different
Stopping a container ends its running process but does not by itself remove the container or its writable layer. A stopped container can therefore retain its instance state while it exists. Deleting the container removes that writable layer. The image layers underneath are unaffected, but ordinary data that existed only in the deleted container layer is gone.
A container is not an image that has been edited
It is more accurate to think of a container as an image plus a separate runtime layer than as a modified copy of the image. That distinction explains why two containers can start from identical files yet diverge safely, and why changes in one do not silently alter future containers made from the same image.
How Docker’s layered and copy-on-write model works
Docker uses a copy-on-write approach to avoid copying every image file for every new container. The exact storage-driver implementation can vary, but the user-visible behavior follows the same model.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errors- Docker assembles the image. Dockerfile instructions produce ordered, read-only layers.
- Docker creates a container. It mounts those image layers as the container’s initial filesystem and adds one empty writable layer above them.
- A read accesses the image directly. If the process only reads a file, Docker can serve it from the existing image layer; no duplicate file is needed in the container layer.
- The first write copies the file up. When the process modifies an existing file, Docker makes a writable copy in the container layer and applies the edit there. The lower, read-only copy remains unchanged.
- New files are written to the top layer. A file that did not exist in the image is created directly in the container’s writable layer.
- Deletion is represented above the image. Removing a file does not physically erase the lower-layer copy. Docker records the removal in the writable layer so that this container’s merged view no longer shows it.
- Each container repeats the process independently. A second container can read the same lower layers, but its first modification creates a separate copy in its own writable layer.
This model is efficient because unchanged files remain shared and read-only. It also explains an important limit: a write-heavy workload can consume substantial space in the writable layer even when the original image is small, because modified files are copied there.
What happens to data: container layer versus volume
Data written to the container layer is tied to that container’s lifetime. Docker documentation recommends volumes for information that must survive container deletion. A volume is stored independently of the container, so removing and recreating the container does not inherently remove the volume’s contents.
Use the container layer for disposable state
Temporary caches, generated scratch files and other state that can be rebuilt are reasonable candidates for the writable layer. If the container is discarded, those files can disappear without harming the application’s source image.
Use a volume for durable application data
Databases, uploaded files, user-generated content and other records that must outlive a particular container should be placed in a volume rather than left only in the writable layer. Volumes are independent of the container and do not increase the size of that writable layer. The application still accesses the mounted data through its filesystem, but the storage lifecycle is no longer coupled to one container.
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Why rebuilding is not a backup strategy
Rebuilding an image recreates the packaged read-only layers; it does not recover files that existed only in a deleted container. Likewise, creating a replacement container from the same image gives you the original image state, not the previous container’s uncommitted runtime changes. Durable data therefore needs its own storage and backup plan.
One image can back many containers
The relationship is one-to-many: a single image can serve as the base for multiple containers. They can share the image’s read-only layers while each receives a private writable layer. Sharing the lower layers avoids storing a full duplicate of the unchanged image for every instance.
Shared starting point, separate changes
Suppose several containers are created from one application image. All begin with the same packaged files. If one container changes a log configuration or generates a temporary file, those changes are visible only in that container’s writable layer. The others retain their own views and can continue from the common base.
Image updates do not mutate existing containers
Building a newer image creates a new image artifact. Existing containers do not silently switch to it; they continue using the image and layers with which they were created. To use the newer build, you create or recreate containers from that image according to your deployment process.
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- Define the application build. A Dockerfile specifies the files and instructions that produce an image.
- Build the image. Docker creates the ordered read-only layers and stores the resulting image.
- Create a container from that image. Docker adds a fresh writable container layer above the image layers.
- Start the container’s process. The process sees a merged filesystem made from the read-only base and its writable top layer.
- Run and modify files as needed. Reads use existing layers; writes, new files and deletions are recorded in the writable layer.
- Stop the container when the process should no longer run. The container and its writable state remain if you keep the container.
- Remove the container when it is no longer needed. Its writable layer is removed, while the image remains available for another container.
- Recreate from an image when you need a fresh instance. A new container starts with the image’s packaged state and a new, empty writable layer, plus any independently retained volumes you attach.
Common misconceptions, corrected
“Is a container just a running image?”
Not exactly. The image supplies the read-only base; the container is the created runtime instance with an additional writable layer. A container may be running or stopped, so “running image” describes only one moment of its lifecycle and leaves out the instance-specific filesystem state.
“If I edit a file in a container, did I edit the image?”
No. The edit is stored in that container’s writable layer. Other containers made from the image do not inherit it, and the image remains unchanged.
“Deleting a container deletes the image, too.”
No. Deleting the container removes its writable layer. The image’s read-only layers are separate and can still be used to create another container.
“A stopped container has no data.”
A stopped container can still have its writable layer and its instance metadata until it is deleted. The data becomes unavailable when that container is removed, unless it was stored in an independent volume.
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No. A volume is separate storage managed independently from the container layer. It is intended for data that should survive replacement or deletion of the container.
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Choosing where a file belongs
| File or state | Best fit | Reason |
|---|---|---|
| Application binaries and dependencies installed during the build | Image layer | They are part of the repeatable, read-only package. |
| Temporary files that can be regenerated | Container writable layer | They can be discarded with the instance. |
| Runtime edits needed only by one instance | That container’s writable layer | The state should remain private to the instance. |
| Database records or uploaded user content | Volume | The data must survive container deletion and recreation. |
| Changes intended for every future instance | A rebuilt image | Packaging the change in a new image makes it repeatable. |
Troubleshooting by symptom
Your changes vanished after replacing a container
The files were probably written only to the old container layer. Put persistent data in a volume, then recreate the container with that volume attached. Rebuilding from the same image cannot restore data that was never stored outside the deleted layer.
Two containers show different versions of a file
Check whether one container modified the file in its writable layer. Containers can share the same image while presenting different merged views after instance-specific writes. If both should have the same packaged content, rebuild the image with the desired file and create new containers from that image.
An image update did not change a running application
Existing containers keep the image layers selected at their creation. Build or obtain the new image, then replace or recreate the container so the process starts from the updated artifact.
Storage usage grows unexpectedly
Look for frequent modifications to large files inside the writable layer. Copy-on-write means the first modification of an image file creates a writable copy, and repeated writes can make the container layer grow. Move durable or write-heavy data to a volume and keep the image focused on relatively stable application files.
How to explain the difference in one sentence
An image is the immutable, layered package; a container is one isolated instance of that package with its own writable layer and lifecycle. The image is the repeatable thing you build and share, while the container is the disposable or persistent-running context in which a process uses it.
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