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An image is a read-only template; a container is a live instance created from that image. Docker builds an image from layered, read-only filesystem content. When Docker creates a container, it adds a separate writable layer above those image layers. The container can create, change, and delete files in that layer without changing the image. Remove the container and its writable layer disappears; the image remains available for another container.
That distinction explains why the same image can run many isolated containers, why changes made during a test can vanish, and why persistent application data belongs in a volume rather than in the container layer.
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Docker image vs. container at a glance
| Aspect | Docker image | Docker container |
|---|---|---|
| What it is | A packaged, layered filesystem and configuration template | An instance created from an image, with a writable container layer |
| Mutability | Read-only when used as a runtime base | Its own top writable layer accepts file changes |
| Dependency | Can exist without any container | Needs an image as its starting point |
| Sharing | The same image layers can underpin many containers | Each container has a separate writable layer and state |
| Data lifetime | Remains until the image is removed | Data in its writable layer is lost when the container is destroyed |
| Typical role | The artifact you build, store, push, or pull | The runtime process and filesystem you create, start, stop, and remove |
Think of the image as the common starting filesystem and the container as one particular runtime copy-on-write view of it. The image does not become a container by itself, and a container does not rewrite the image it came from.
What a Docker image contains
A layered, read-only filesystem
An image is assembled from filesystem layers. Instructions in a Dockerfile contribute layers as the image is built. Those layers are treated as read-only when the image is used to create containers. A later build can produce a new image, but an existing image is not edited by a running container.
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Layering lets several images share identical lower layers. If two images contain the same base operating-system files, Docker can keep one shared copy of those files instead of storing a complete duplicate for each image. The image therefore represents a reusable artifact rather than a mutable workspace.
An image can exist on its own
You can store an image without creating a container from it. Images can be kept locally, transferred to another machine, or pushed to and pulled from an image registry. No process has to be running for the image to exist. This is why teams can prepare and distribute an image before any particular deployment creates a container.
What an image does not contain
An image does not contain the changing runtime state of every container that uses it. A log file written by one container, a temporary file created during a test, or a modified configuration file belongs to that container’s writable layer, not to the image. If you need that change to become part of a reusable artifact, the normal conceptual solution is to build a new image rather than treating a running container as the image itself.
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What a Docker container adds
An instance plus a writable layer
Creating a container starts with the image’s read-only layers and adds a new writable “container layer” on top. Reads that do not require a change can come directly from the image. New files and modifications are recorded in the container layer, giving that one container its own state while leaving the shared image untouched.
The container is the runtime unit in which an application process operates. It has its own view of the image filesystem and its own writable changes. Another container created from the same image receives a different writable layer, so the two containers do not automatically see each other’s changes in that layer.
A container depends on an image
An image can sit unused, but a container needs an image as its starting point. The image supplies the files and configuration that the container receives at creation time; the container supplies the per-instance writable area and runtime state. Calling a container a “running image” is therefore a useful shortcut only if you remember that the container is the image plus an additional writable layer and runtime instance.
How copy-on-write makes the relationship efficient
Docker uses a copy-on-write approach for the container layer. At first, a container can read files from the image layers without making a private copy of every file. When the container first modifies a particular file, Docker copies that file into the writable layer and applies the change there. Unchanged files continue to be read from the image.
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- First modification: The file is copied into the container’s writable layer.
- Further writes: Subsequent changes to that file affect the writable copy, not the image copy.
- Deletion: The container records the deletion in its writable layer while the original image content remains available to other containers.
This arrangement avoids duplicating the entire image for every container. It also explains an important storage behavior: a container can begin with a small amount of private data, but modifying many large files can make its writable layer grow.
What happens when a container is deleted
Deleting a container removes its writable layer. Any file that existed only there—whether created by the application or copied up during a modification—goes away with the container. The underlying image layers are unchanged and can still be used to create a replacement container.
This is intentional rather than an error. Disposable container state makes it practical to replace an instance with a fresh one from the same image. It also means that treating the writable layer as permanent storage is risky: a rebuild, replacement, or cleanup operation can remove data that was never stored elsewhere.
Volumes are different from the container layer
Data that must survive container deletion should be placed in a volume or another storage mechanism independent of the container. A volume is separate from the container’s writable layer, so its contents can remain when the container is removed and can be attached to a replacement container.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsVolumes also do not increase the size of the writable container layer. That separation is useful for state such as user-generated files, databases, caches that must outlive a process, or generated media. The exact application determines which files are disposable and which require independent storage, but the storage rule is constant: data in the container layer is ephemeral; persistent data belongs outside it.
One image can back many containers
The image-to-container relationship is one-to-many. Suppose an image contains an application and its dependencies. Docker can create several containers from that single image. All of them can read the common image layers, while each receives an independent writable layer.
| Object | Shared with other containers? | Where its changes go |
|---|---|---|
| Image layers | Yes, when containers use the same image | They do not change during container operation |
| Container A’s writable layer | No; it belongs to container A | Changes made by container A |
| Container B’s writable layer | No; it belongs to container B | Changes made by container B |
| Attached volume | It can be reused by containers when deliberately attached | Data stored independently of either container layer |
If container A edits a file that originally came from the image, container B still sees the image version unless B makes its own change or both containers deliberately use shared external storage. Removing A does not alter B’s writable layer or the image.
The image-to-container lifecycle
The following sequence shows where each kind of change belongs.
- Define the application content. A Dockerfile and its build context describe the files, dependencies, and instructions that form an image.
- Build the image. Docker produces a layered, read-only artifact. The result can remain stored without any container.
- Create a container. Docker selects that image and adds a new writable container layer for this instance.
- Run the application. Reads use the image layers when possible; new files and modifications are written to the container layer.
- Persist required data separately. Files that must survive replacement are written to a volume or another independent store rather than relying on the writable layer.
- Replace or remove the container. Destroying the container removes its writable layer. The image and any independent volume remain available.
- Create a fresh instance when needed. A new container starts from the unchanged image, then receives its own writable layer and any explicitly attached persistent data.
The practical boundary is clear: image-building changes belong in the image definition and a new build; per-instance runtime changes belong in the container layer; durable application data belongs in independent storage.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common misconceptions, corrected
“The container changes the image.”
It does not. A modified file is copied into the container’s writable layer. Other containers using the same image continue to use the original image content. To distribute a deliberate change, create a new image rather than expecting a running container’s edits to become a shared base.
“Deleting a container deletes the image.”
Container deletion removes the container and its writable layer. The image is a separate object and remains unless it is removed separately.
“Every container gets a full copy of the image.”
Not normally. Copy-on-write allows containers to share unchanged image layers. A container gets private storage for changes, not an automatic duplicate of every unchanged file.
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Only data stored outside the container layer has an independent lifetime. Files written solely into the writable layer disappear when that container is destroyed. Use a volume or another external store for data that must survive.
Best Value
“An image is the process that is running.”
An image is a static, read-only template. The process runs in a container created from that template. A stopped container can still retain its writable layer until it is removed, while the image can exist with no containers at all.
“A container is the unit I should share with a teammate.”
For repeatable distribution, the image is the reusable artifact. A container contains instance-specific state and may include ephemeral changes. Sharing the image and separately managing persistent data gives another environment a reproducible starting point.
Choosing where a change belongs
| Change or data | Best home | Reason |
|---|---|---|
| Installing an application dependency for every instance | Image definition and a rebuilt image | It should be present consistently whenever a new container is created |
| A temporary file generated during one run | Container writable layer | It is instance-specific and can be discarded |
| User uploads or records that must survive replacement | Independent volume or external storage | The container layer is removed with the container |
| A one-off experiment that should not affect other instances | That container’s writable layer | Copy-on-write isolates the experiment from the image and sibling containers |
| A configuration value that differs per deployment | Runtime configuration or an attached data source | It should not require mutating the shared image for one instance |
What to check when data or changes disappear
- Confirm what was removed. If the container was destroyed, its writable layer was destroyed with it. The image may still be present, but it does not contain the lost runtime edits.
- Separate reproducible content from state. Put software and repeatable setup into the image build; put durable application data into a volume or another independent store.
- Check whether a file was modified. A file that was only read may still come from the image. A file that was changed belongs to the container layer and will not automatically appear in a replacement container.
- Check each container independently. Containers made from one image do not share their writable layers. If two instances need common data, attach an explicitly shared external store.
- Recreate from the correct image. A replacement container starts from the image’s unchanged layers, then receives only the persistent data and runtime configuration that you attach deliberately.
Why the distinction matters in practice
Keeping images immutable and containers replaceable makes deployments easier to reason about. Every new container starts from the same packaged baseline, while copy-on-write limits private storage to files that actually change. The trade-off is that accidental writes to the container layer are fragile: they are useful for transient work but unsuitable as the only copy of important data.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThe distinction also clarifies troubleshooting. If every new container is missing a dependency, the fix belongs in the image build. If only one instance has an unexpected file, inspect that container’s writable state. If data must outlive the instance, inspect the volume or external store rather than the image. These are different failure domains, and treating them as interchangeable leads to lost data or inconsistent deployments.
Is a container just a running image?
Not technically. An image is the read-only, layered template. A container is an instance created from that image, with a writable layer and its own runtime state. The container may be running or stopped, but in either case it remains distinct from the image. Saying “running image” can communicate the general idea to a beginner, yet it hides the writable layer—the part that determines isolation, copy-on-write behavior, and data loss on deletion.
The rule to remember
Build repeatable software into an image, run that image as one or more containers, and keep any data that must survive outside the container’s writable layer. One image can safely serve many containers because the read-only layers are shared while each instance receives private copy-on-write storage. Deleting an instance removes its private changes, not the image; persistent volumes remain independent when they are used deliberately.
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