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A Docker container can serve files that were copied into it after its image was built, while the image itself still contains older software. Restarting that container may preserve those changes; replacing it from the unchanged image can discard them. The reliable release workflow is to rebuild the image and recreate the service, then verify the artifact and processes that actually run.
Why a container can run software absent from its image
A Docker image provides the filesystem layers used to create a container. A running container can also accumulate filesystem changes of its own, including files copied into it after the image was built. Those changes do not automatically become part of the image. Docker explains the distinction between image layers and a container’s writable layer in its guide to understanding image layers.
That distinction explains two different outcomes. Restarting the same container can leave its container-level changes in place. Replacing it with a new container from the old image starts from the image’s contents instead, so files that existed only in the prior container may disappear.
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| Operation | What it does | What it does not guarantee |
|---|---|---|
| Restart a container | Stops and starts that container again. | It does not turn container-only changes into image contents. |
| Recreate a service container | Creates a replacement container. Compose can stop and recreate existing containers when service configuration or the image changes; --force-recreate forces recreation even when image and configuration have not changed. |
It does not rebuild the image. See Docker’s Compose up reference. |
| Build an image | Produces an image from the build inputs and instructions. | It does not, by itself, replace a currently running service container. |
Docker’s Compose create reference describes container creation. The key operational point is that rebuilding an image and recreating a container are separate actions: doing only one may leave the running service on the wrong artifact.
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For a code release, rebuild and recreate the service
Docker’s production guidance is explicit: “When you make changes to your app code, remember to rebuild your image and recreate your app’s containers.” For its web example, the documented sequence is:
docker compose build web
docker compose up --no-deps -d webThe first command builds the service image. The second recreates and starts the service without starting its dependencies. Follow the same pattern for the service name in your Compose configuration, and account for your deployment’s dependency and rollout needs. See Use Compose in production.
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Do not treat docker compose up --force-recreate as a substitute for a build. It can replace the container while still using an image that lacks the intended program or code. Conversely, rebuilding without replacing the running service does not ensure that service begins using the new image.
When file sync is appropriate—and when it is not
Compose Watch provides distinct development behaviors. Its sync action copies changed files into a running container, while rebuild builds a new image and replaces the service container. Docker documents these choices in Use Compose Watch and the Compose Develop Specification.
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- Use sync for development when you want selected file edits copied into an existing container as part of a development workflow.
- Use rebuild for release changes when the change must be represented in the image used to deploy the service.
Syncing files into a live container can be useful, but it is not equivalent to producing a new image. If a later deployment recreates the container from an image that never received those changes, the synced or copied files may be lost.
Check what is running, not just whether it responds
A health check or successful request establishes that some service is responding; it does not establish that the service is serving the intended build or running every required program. Two practical checks are to compare the identity of the served artifact with the intended build and to compare the running program list with the configuration’s expected processes.
- Verify artifact identity: compare a served bundle hash or other build identifier with the value associated with the release you intended to deploy.
- Verify expected processes: compare the programs reported by the container with the programs required by its configuration.
These are operational checks, not Docker-native guarantees. In an account published by Nebosa Company on 2026-09-22, a changed served bundle hash exposed a rollback after a container was recreated. The same account reported an image with 19 supervisord programs where 20 were expected, with cert-watch missing. The publisher’s account is not an independently audited investigation, so those figures describe that incident, not a general Docker failure rate. Read the account on DEV Community.
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- Identify the deployed image and build. Establish which image the service is configured to use and what build identity the intended release should serve.
- Check for container-only changes. Determine whether the missing files were copied or synced into the running container instead of being included in the image build.
- Build the intended image. Use
docker compose build SERVICE, replacingSERVICEwith the Compose service name. - Recreate the service from that image. Use the deployment-appropriate Compose command; Docker’s production example uses
docker compose up --no-deps -d webafter building. - Verify the result. Check the served artifact identity and compare actual processes with the expected configuration.
If the service still reports an old artifact or lacks a required process, investigate the build inputs, service image configuration, and process configuration. A successful container recreation alone does not show that the correct artifact was built or selected.
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