Let us examine the error docker error failed to compute the cache key, the reasons, and the most effective troubleshooting steps to avoid the error with the support of our Docker hosting support services at Bobcares.

Fix Docker Error: Failed to Compute Cache Key (Step-by-Step Guide)

The Docker failed to compute cache key problem occurs when the docker buildcommand is called in a directory that does not contain the Dockefile.

Error

parent-directory

- test-file-1.py

- child-directory (child directory with Dockerfile)

-DockerFile

- test-file-2.py

The Dockerfile can be found in the child directory of the directory structure shown above.

Understanding the Error

The “Docker failed to compute cache key” error usually appears during the Docker build process. It indicates that Docker is unable to generate a cache key for the build context, which is essential for optimizing the build by reusing layers from previous builds. Without a valid cache key, the build process may fail or slow down significantly.

This issue can stem from improperly executed docker build commands, similar to what’s seen in errors like Docker error removing network active endpoints.

Causes of the Error

A common reason for the “failed to compute cache key” error is running the `docker build` command in a directory that doesn’t contain the Dockerfile. Here’s a closer look at why this happens:

• Build Context:

  When we execute the `docker build` command, Docker uses the specified directory as the build context, the set of files and folders it can access during the build. It expects to find a Dockerfile within this context, as the Dockerfile contains the instructions needed to build the image.
• Missing Dockerfile:

  If the Dockerfile is missing from the build context, Docker can’t proceed. Since it relies on the Dockerfile to compute cache keys and identify reusable layers, its absence disrupts the entire build process, leading to the “failed to compute cache key” error.
• Error Diagnosis:

  This error signals that Docker couldn’t locate the Dockerfile or gather the necessary data to compute the cache key. Without this, Docker cannot optimize or continue the build, resulting in the error message.

Solutions

Method 1. Use the Correct Dockerfile Path

Considering the directory layout above, the first error was that we were on the parent directory and expecting to run docker-build successfully, but the Dockerfile was located in the child directory.

Here is the incorrect directory:

 $ pwd

/home/rwagh/my-project/parent-directory

The following is the error that we will receive after running the docker-build command –

failed to compute cache key: "/parent-directory/child-directory/test-file-2.py" not found: not found

We now understand the issue and problem linked to the way the docker-build command is being executed. So our goal is to run the Dockerfile located in the child directory. Follow the troubleshooting steps given below:

1. Firstly, we have to navigate and switch to the parent directory –

   cd /home/rwagh/my-project/parent-directory
2. Following that, we must run the following docker-build command, specifying the Docker file in the child directory:

   docker build -f child-directoryDockerfile

So, the command above will assist us in resolving the docker error that failed to compute the cache key. The only thing that was lacking was to indicate the exact path of the Dockerfile that was already in the correct directory.

It is usually suggested to use the explicit Dockerfile path to avoid issues like those encountered in Docker container attach and detach scenarios, where context matters greatly.

Note: It is usually suggested to use the explicit docker-file path to avoid issues caused by Visual Studio’s fuzziness.

If the issue still occurs after the above steps, consider the following:

• First, make sure the .dockerignore file doesn’t exclude the files required during the build. A misconfigured or overly restrictive .dockerignore can prevent Docker from accessing necessary files, leading to cache key computation failures.
• If we use Docker within Visual Studio or run Docker on Windows, environment-related issues might be a factor. Switching to a Linux-based Docker environment often resolves such issues. We can do this by configuring Docker to use the WSL backend and rerunning the build steps in a terminal.

Method 2. Look for the Missing Directory

If the previous procedure does not assist to resolve the issue, carefully examine the logs and go through the errors.

Another error may appear in the logs stating that test-file-2.py is missing.

failed to compute cache key: "/child-directory/test-file-2.py" not found: not found

So the issue that we can see over here is either of these two:

1. Either the entire directory is missing or it is incomplete. – test-file-2.py in /child-directory
2. Or we’re missing something. test-file-2.py is a Python script.

To avoid this problem, we must ensure that the files in the project directory are checked.

Method 3. Confirm Build Context and Dockerfile Location

To ensure a successful build, confirm that the Dockerfile is in the directory we are executing the build from, or specify its path:

docker build -f /path/to/Dockerfile -t my-image .

Then, make sure the context directory (the . in the command above) includes all referenced files and folders.

If we are using an outdated version of Docker, we have to update it. The older version it may contain bug fixes that address cache-related issues.

Also, check for Docker version-related bugs. Errors such as those in Docker containerconfig failures can sometimes be resolved with a version upgrade.

Method 4. Review Your .dockerignore File

Misconfigured .dockerignore rules can cause Docker to ignore files required for the build, resulting in cache key errors.

For example:

*

!dist/

This configuration excludes all files except the dist/ folder. If the Dockerfile needs other files, they must be explicitly allowed:

COPY nginx.conf.spa /etc/nginx/nginx.conf

If nginx.conf.spa isn’t whitelisted in .dockerignore, the build will fail.

Method 5. Understand Visual Studio’s Build Behavior

Visual Studio often runs Docker build from the solution folder and specifies the Dockerfile using the—f option, which may lead to path-related errors.

Here is an example project structure:

\WorkerService2          # Solution folder

├── WorkerService2.sln

└── WorkerService2     # Project folder

├── Dockerfile

└── WorkerService2.csproj

Here is the correct way to build:

cd \WorkerService2

docker build -f WorkerService2/Dockerfile --force-rm -t worker2/try7 . 

We can use Detailed verbosity in Visual Studio build output to understand what paths it’s using during Docker operations.

Method 6. Use Linux Environment (WSL)

If we are using Docker with Windows, especially inside Visual Studio, environment conflicts can occur. Switching to Linux (WSL) often helps. This is especially helpful if you run into complex networking issues like in Docker error: server misbehaving. So, change Docker’s environment to use WSL.

We can run your build from the terminal:

docker build -t containername/tag -f ProjectFolder/Dockerfile .

docker run containername/tag

Method 7. Validate Directory References in COPY Commands

Errors can also be caused by referencing files or folders that don’t exist in the build context:

COPY /.ssh/id_rsa.pub /.ssh/

This fails if .ssh/id_rsa.pub doesn’t exist in the build context. Ensure all paths in your COPY and ADD commands are valid and relative to the build context.

Method 8. Double-Check .dockerignore Again

Docker restricts access to files outside the build context for security reasons. If necessary files are excluded in .dockerignore, we will run into this error.

Here is an example fix:

COPY ./script_file.sh /

RUN /script_file.sh

Make sure script_file.sh is included in the context and not ignored.

Other Troubleshooting Options

• Examine the.dockerignore file. It may overlook the required files for the copy operation, resulting in the failure to compute the cache key error.
• If we set the Docker environment to Windows while adding Docker support, we can run into the same problem. Even executing it in Visual Studio resulted in issues.

  As Docker runs in the Windows Subsystem for Linux, we can change the environment to Linux in this scenario (WSL), after that, move back to the terminal to run the commands. We can remove the error by moving the Solutions or the Root folder.

After that, we can do the Docker build like this:

docker build -t containername/tag -f ProjectFolder/Dockerfile

.

Then type in the 

 docker run

:

docker run containername/tag

This will remove the error.

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Conclusion

To conclude we have now gone through the docker error failed to compute cache key error along with the possible causes and the Troubleshooting tips to effectively remove the error within a few simple steps with the assistance of our Docker hosting support services.