Docker: Troubleshooting 6.X Images With Arbitrary Users

Dec 2, 2025 by Alex Johnson 56 views

Troubleshooting Docker: Why 6.X Images Fail with Arbitrary Users

Introduction

Are you encountering issues running Docker 6.X images with the --user option? You're not alone. Many users have reported similar problems when trying to specify a user ID (UID) or group ID (GID) when running Docker containers. In this comprehensive guide, we'll delve into the reasons behind this issue, explore potential solutions, and provide a step-by-step approach to resolve the "Permission denied" error. This article aims to provide a clear understanding of the problem and equip you with the knowledge to overcome it.

When working with Docker, the ability to run containers under specific user accounts is crucial for security and proper file system permissions. The --user flag in the docker run command allows you to specify the user context within the container. However, when attempting to use this flag with certain images, particularly those based on the 6.X version, you might encounter the frustrating Errno::EACCES: Permission denied error. This error typically arises due to file permission issues within the container's file system. Let's break down the problem and explore effective solutions.

This issue often stems from discrepancies in how file permissions are handled within the Docker image and the host system. When you specify a user using the --user flag, Docker attempts to run the container's processes under that user. If the necessary files and directories within the container do not have the appropriate permissions for the specified user, you will encounter permission errors. This is particularly common with images that create or modify files at runtime, such as those used for web applications or databases. Therefore, understanding the underlying causes and implementing the correct fixes is essential for a smooth Docker experience.

Understanding the Problem

To fully grasp the issue, let's dissect the error message and the context in which it occurs. The error message Errno::EACCES: Permission denied @ rb_sysopen - /usr/src/redmine/public/assets/.manifest.json indicates that the user within the container does not have the necessary permissions to access the /usr/src/redmine/public/assets/.manifest.json file. This typically happens when the file is owned by a different user or group, or when the permissions are set such that the specified user cannot read or write to the file.

In the provided scenario, the user is attempting to run a Redmine 6.X image with the --user 4042:4043 option, which sets the user ID to 4042 and the group ID to 4043. The error occurs during the rake aborted! process, specifically when the application tries to write to the .manifest.json file. This file is part of the asset compilation process in Ruby on Rails applications like Redmine, and it requires write permissions for the user running the application. The fact that running the same image without the --user option or running an older 5.X image with the --user option works suggests that the file permission issue is specific to the 6.X image and the way it handles user contexts.

The core issue is that the user context under which the application runs inside the container does not have the required permissions to access or modify certain files and directories. This can be due to several factors, such as incorrect file ownership within the image, misconfigured volume mounts, or changes in the way newer versions of the application handle file permissions. By examining these potential causes, we can identify the most effective strategies for resolving the problem and ensuring that the application runs smoothly under the specified user context.

Diagnosing the Permission Issue

Before implementing any solutions, it's crucial to accurately diagnose the root cause of the permission issue. Here are several steps you can take to gain a better understanding of the problem:

Inspect File Permissions Inside the Container: Use the docker exec command to enter the running container and inspect the file permissions of the problematic files and directories. For example:
```
docker exec -it <container_id> bash
ls -l /usr/src/redmine/public/assets/.manifest.json
```
This will show you the owner, group, and permissions of the file. Compare these permissions with the user ID and group ID you are using with the --user flag. If the file is owned by a different user or group, or if the permissions do not allow the specified user to write to the file, this is likely the cause of the issue.
Check User and Group IDs: Verify that the user and group IDs you are using with the --user flag exist within the container. You can use the id command inside the container to check this:
```
docker exec -it <container_id> bash
id 4042
id 4043
```
If the user or group does not exist, you may need to create them or use a different user ID and group ID.
Examine Dockerfile and Entrypoint Scripts: Review the Dockerfile and entrypoint scripts of the image to see how file permissions are being set and how the application is being started. Look for any commands that change file ownership or permissions, such as chown or chmod. Also, check how the application is being run and whether it is explicitly setting a user context. This step is crucial as it helps identify any potential misconfigurations in the image itself.
Test with a Minimal Example: Try running a simple command inside the container with the --user flag to isolate the issue. For example:
```
docker run --user 4042:4043 <image_name> touch /tmp/testfile
```
If this command fails with a permission error, it indicates that the issue is not specific to the Redmine application but rather a general problem with running the container under the specified user context. Successfully isolating the issue will guide you to the appropriate solution.

By systematically diagnosing the issue, you can pinpoint the exact cause of the permission error and implement the most effective solution. Understanding the file permissions, user contexts, and how the application interacts with the file system is key to resolving these types of problems.

Solutions to Resolve the Issue

Once you've diagnosed the problem, you can implement one of the following solutions to resolve the Permission denied error when running Docker 6.X images with an arbitrary user:

Adjust File Ownership and Permissions: The most common solution is to adjust the ownership and permissions of the files and directories that the application needs to access. You can do this either within the Dockerfile when building the image or at runtime using docker exec. If you choose to do it in the Dockerfile, add the following commands:
```
USER root
RUN chown -R 4042:4043 /usr/src/redmine/public/assets
RUN chmod -R 755 /usr/src/redmine/public/assets
USER 4042:4043
```
Here, chown changes the ownership of the /usr/src/redmine/public/assets directory to the specified user and group, and chmod sets the permissions to allow the user to read, write, and execute files in that directory. If adjusting permissions at runtime:
```
docker exec -it <container_id> chown -R 4042:4043 /usr/src/redmine/public/assets
docker exec -it <container_id> chmod -R 755 /usr/src/redmine/public/assets
```
This ensures that the user context under which the application is running has the necessary permissions to access the files.
Use Volume Mounts with Correct Permissions: If you are using volume mounts to persist data between the container and the host, ensure that the mounted directories on the host have the correct permissions. For example, if you are mounting a directory from the host to /usr/src/redmine/public/assets in the container, make sure the directory on the host is owned by the same user and group as the user inside the container. You can use the chown command on the host to change the ownership:
```
sudo chown -R 4042:4043 /path/on/host/redmine_assets
```
This step is particularly important when dealing with persistent storage, as incorrect host-side permissions can lead to runtime errors within the container.
Create User and Group Inside the Container: If the user and group IDs you are using with the --user flag do not exist inside the container, you need to create them. You can add the following commands to the Dockerfile:
```
RUN adduser -D -u 4042 redmine_user
RUN addgroup -g 4043 redmine_group
RUN usermod -a -G redmine_group redmine_user
```
This creates a new user and group with the specified IDs. Ensure that you adjust the file ownership and permissions accordingly after creating the user and group.
Modify Entrypoint Script: If the entrypoint script is setting file permissions or user contexts, you may need to modify it to correctly handle the --user flag. For example, if the entrypoint script is hardcoding a specific user to run the application, you can modify it to use the user specified with the --user flag. Reviewing and adjusting the entrypoint script can be essential, especially when dealing with complex application setups.
Use gosu or su-exec: These tools allow you to easily switch users within a running container. You can install gosu or su-exec in your Docker image and use them in your entrypoint script to run the application under the specified user. For example, if you're using gosu:
```
RUN apt-get update && apt-get install -y gosu
COPY docker-entrypoint.sh /docker-entrypoint.sh
ENTRYPOINT ["/docker-entrypoint.sh"]
```
And in your docker-entrypoint.sh:
```
#!/bin/bash
set -e

# Your setup commands here

gosu 4042:4043 bundle exec rails server -b 0.0.0.0
```
This ensures that the application is run under the correct user context.

By implementing one or more of these solutions, you can effectively resolve the Permission denied error and run your Docker 6.X images with an arbitrary user. Remember to carefully diagnose the issue before applying any fixes to ensure you are addressing the root cause of the problem.

Best Practices for Running Docker Containers with Arbitrary Users

To avoid permission issues and ensure the smooth operation of your Docker containers, it's crucial to follow best practices when running containers with arbitrary users. Here are some key guidelines to keep in mind:

Use Non-Root Users: Always strive to run your containers with non-root users. Running containers as root poses significant security risks, as it grants the container full access to the host system. By creating and using non-root users, you minimize the potential impact of security vulnerabilities.
Principle of Least Privilege: Apply the principle of least privilege by granting the container user only the necessary permissions. Avoid giving the user excessive permissions that could be exploited in case of a security breach. This involves carefully reviewing the application's requirements and granting only the required access to files and directories.
Define Users and Groups in the Dockerfile: Define the necessary users and groups in your Dockerfile to ensure consistency and reproducibility. This simplifies the process of building and deploying containers across different environments. Use commands like adduser and addgroup to create users and groups with specific IDs, ensuring they match the expectations of your application.
Set File Ownership and Permissions Appropriately: Carefully set file ownership and permissions within the container to align with the user context. Use chown and chmod commands to ensure that the necessary files and directories are accessible to the container user. Pay close attention to the application's file system requirements and set permissions accordingly.
Leverage Volume Mounts Securely: When using volume mounts to share data between the host and the container, ensure that the host-side directories have appropriate permissions. Match the ownership and permissions on the host to the user context within the container. This is crucial for maintaining data integrity and preventing permission-related issues at runtime.
Regularly Update Base Images: Keep your base images up-to-date to benefit from security patches and bug fixes. Outdated base images may contain vulnerabilities that can be exploited by malicious actors. Regularly updating base images ensures that your containers are built on a secure foundation.
Scan Images for Vulnerabilities: Use vulnerability scanning tools to identify and address potential security issues in your Docker images. These tools can detect vulnerabilities in the base image and application dependencies, helping you proactively mitigate security risks. Incorporate vulnerability scanning into your CI/CD pipeline for continuous security assessment.
Implement a Robust Logging and Monitoring Strategy: Implement a comprehensive logging and monitoring strategy to track the behavior of your containers. Monitor container logs for error messages and security-related events. This helps you quickly identify and address potential issues before they escalate into major problems.

By adhering to these best practices, you can significantly reduce the risk of permission issues and security vulnerabilities when running Docker containers with arbitrary users. A proactive and security-conscious approach is essential for maintaining a stable and secure Docker environment.

Conclusion

Running Docker containers with arbitrary users is a powerful feature that enhances security and flexibility. However, it can also lead to permission issues if not handled correctly. By understanding the root causes of these issues and implementing the solutions discussed in this guide, you can effectively troubleshoot and resolve the Permission denied error. Remember to follow best practices for running containers with non-root users to ensure a secure and efficient Docker environment.

By carefully diagnosing the problem, adjusting file ownership and permissions, using volume mounts correctly, and following best practices, you can seamlessly run your Docker 6.X images with arbitrary users. This ensures that your applications operate securely and efficiently within the containerized environment.

For more information on Docker security best practices, visit the Docker Documentation.