Dockerでボリュームをマウントする際の「permission denied」エラーの解決方法

Introduction

Docker is a powerful containerization platform that allows developers to package and deploy applications easily. One common issue that users encounter is the 'permission denied' error when mounting volumes in Docker. This error occurs when the container does not have the proper permissions to access files or directories on the host machine.

In this lab, you will learn how to identify, troubleshoot, and resolve permission denied errors when working with Docker volumes. By the end of this tutorial, you will understand how Docker volumes work, how permissions affect them, and the best practices for setting up volumes with the correct permissions.

Skills Graph

Understanding Docker Volumes

Docker volumes are a mechanism for persisting data generated and used by Docker containers. They allow you to store data independently of the container's lifecycle, making it easier to back up, share, and manage your application data.

Let's start by exploring Docker volumes and creating a basic volume to understand how they work.

What are Docker Volumes?

Docker volumes serve several important purposes:

• They persist data even when containers are removed
• They allow sharing data between containers
• They separate storage management from container management
• They provide better performance than writing to the container's writable layer

Creating and Managing Docker Volumes

First, let's create a simple Docker volume:

docker volume create my_volume

To list all volumes:

docker volume ls

You should see output similar to:

DRIVER    VOLUME NAME
local     my_volume

Let's inspect our newly created volume to see where it's stored on the host machine:

docker volume inspect my_volume

The output will show details about the volume:

[
  {
    "CreatedAt": "2023-XX-XX....",
    "Driver": "local",
    "Labels": {},
    "Mountpoint": "/var/lib/docker/volumes/my_volume/_data",
    "Name": "my_volume",
    "Options": {},
    "Scope": "local"
  }
]

The Mountpoint is where Docker stores the volume data on the host system.

Testing Volume Mounting

Let's run a container that mounts our volume and write some data to it:

docker run --rm -v my_volume:/data alpine sh -c "echo 'Hello from Docker!' > /data/test.txt"

This command:

• Creates a temporary Alpine Linux container with the --rm flag (it will be removed when it exits)
• Mounts our my_volume to the /data directory inside the container
• Writes "Hello from Docker!" to a file named test.txt in the volume

Now, let's verify the data persists by reading it from another container:

docker run --rm -v my_volume:/data alpine cat /data/test.txt

You should see:

Hello from Docker!

This demonstrates how Docker volumes persist data across different containers.

Creating a Permission Denied Scenario

Now that we understand basic Docker volume usage, let's create a scenario that produces the 'permission denied' error. This will help us understand what causes the issue and how to resolve it.

Setting Up a Test Directory

First, let's create a directory on the host machine and a file with specific permissions:

mkdir -p ~/project/docker-test
echo "This is a test file." > ~/project/docker-test/testfile.txt
chmod 700 ~/project/docker-test/testfile.txt

These commands:

1. Create a directory called docker-test in your project folder
2. Create a test file with some content
3. Set permissions on the file to be readable, writable, and executable only by the owner (you)

Let's check the permissions of the file:

ls -la ~/project/docker-test/

You should see output similar to:

total 12
drwxr-xr-x 2 labex labex 4096 XXX XX XX:XX .
drwxr-xr-x X labex labex 4096 XXX XX XX:XX ..
-rwx------ 1 labex labex   19 XXX XX XX:XX testfile.txt

Notice that the file permissions are set to 700 (-rwx------), which means only the owner (you) can read, write, or execute the file.

Encountering the Permission Denied Error

Now, let's try to access this file from within a Docker container:

docker run --rm -v ~/project/docker-test:/app ubuntu cat /app/testfile.txt

You should see an error message similar to:

cat: /app/testfile.txt: Permission denied

This is because Docker containers, by default, run as the root user inside the container, but that root user doesn't map to the same user ID as your host user. When Docker mounts a host directory, the permission checks are still applied based on the original file permissions and user IDs.

Understanding the Problem

The permission denied error occurs because:

1. The file on your host is owned by your user (labex)
2. The file has permissions set to 700 (only the owner can access it)
3. The Docker container runs as a different user ID (usually root, which is UID 0)
4. Even though the container user is "root", it doesn't have the same privileges as the host root user when accessing mounted volumes

Let's verify the user IDs to understand this better:

echo "Host user ID: $(id -u)"
docker run --rm ubuntu bash -c "echo Container user ID: \$(id -u)"

This shows that while you're running as your user ID on the host (likely 1000), the container is running as user ID 0 (root). Despite being "root" inside the container, when accessing host-mounted files, the container's root user is still subject to the host's permission checks.

Resolving Permission Denied Errors

Now that we understand the cause of the permission denied error, let's explore several methods to resolve it.

Method 1: Modifying File Permissions on the Host

The simplest approach is to change the permissions of the files on your host to allow other users to access them:

chmod 755 ~/project/docker-test/testfile.txt

This changes the permissions to 755 (-rwxr-xr-x), allowing anyone to read and execute the file, but only the owner can modify it.

Let's try accessing the file from a container again:

docker run --rm -v ~/project/docker-test:/app ubuntu cat /app/testfile.txt

Now you should see the content of the file:

This is a test file.

This works because the file is now readable by "others" on your host system, which includes the container's user.

Method 2: Using the --user Flag

Another approach is to tell Docker to run the container with the same user ID as your host user:

## Reset the file permissions to be restrictive
chmod 700 ~/project/docker-test/testfile.txt

## Get your user ID and group ID
USER_ID=$(id -u)
GROUP_ID=$(id -g)

## Run the container with your user ID
docker run --rm --user $USER_ID:$GROUP_ID -v ~/project/docker-test:/app ubuntu cat /app/testfile.txt

You should now be able to read the file content despite its restrictive permissions:

This is a test file.

This works because:

1. We run the container with the same user ID as your host user
2. The permissions on the file allow access to that user ID
3. Docker passes the user ID through to the container's processes

The --user flag is particularly useful when you need to maintain restrictive permissions on your host files.

Method 3: Adjusting Owner and Group IDs

Let's create a new file owned by a different user to demonstrate this method:

## Create a file as root
sudo bash -c 'echo "This is a root-owned file." > ~/project/docker-test/rootfile.txt'
sudo chown root:root ~/project/docker-test/rootfile.txt
sudo chmod 600 ~/project/docker-test/rootfile.txt

## Let's see what we have
ls -la ~/project/docker-test/

The output should show:

total 16
drwxr-xr-x 2 labex labex 4096 XXX XX XX:XX .
drwxr-xr-x X labex labex 4096 XXX XX XX:XX ..
-rw------- 1 root  root    25 XXX XX XX:XX rootfile.txt
-rwx------ 1 labex labex   19 XXX XX XX:XX testfile.txt

Now try to access the root-owned file from a container running as root:

docker run --rm -v ~/project/docker-test:/app ubuntu cat /app/rootfile.txt

You should see the content:

This is a root-owned file.

This works because:

1. The container runs as root (UID 0) by default
2. The file is owned by root (UID 0) on the host
3. The permissions (600) allow the owner to read the file

This demonstrates that the actual user IDs matter, not just the names. When the container's user ID matches the file's owner ID, the permission checks will succeed if the owner has the necessary permissions.

Best Practices for Docker Volume Permissions

Now that we understand how to resolve permission issues, let's discuss some best practices for setting up Docker volumes with proper permissions.

Using Named Volumes

Named volumes are managed by Docker and typically have better permission handling than bind mounts. Let's create a named volume and see how it behaves:

## Create a named volume
docker volume create data_volume

## Write to the volume as root in a container
docker run --rm -v data_volume:/data ubuntu bash -c "echo 'Created by root user' > /data/rootfile.txt && ls -la /data"

## Read from the volume as a non-root user
docker run --rm --user 1000:1000 -v data_volume:/data ubuntu cat /data/rootfile.txt

You'll notice that both operations work without permission issues. This is because Docker handles the permissions differently for named volumes than for bind mounts.

Creating a Consistent Development Environment

For development environments, it's often useful to set up a consistent user ID between your host and container:

## Create a Dockerfile for a development container
mkdir -p ~/project/dev-container
cat > ~/project/dev-container/Dockerfile << EOF
FROM ubuntu:22.04

ARG USER_ID=1000
ARG GROUP_ID=1000

RUN apt-get update && apt-get install -y sudo

## Create a non-root user with the same ID as the host user
RUN groupadd -g \${GROUP_ID} developer && \\
    useradd -u \${USER_ID} -g \${GROUP_ID} -m -s /bin/bash developer && \\
    echo "developer ALL=(ALL) NOPASSWD:ALL" > /etc/sudoers.d/developer

USER developer
WORKDIR /home/developer

CMD ["bash"]
EOF

## Build the development container
cd ~/project/dev-container
docker build -t dev-container .

Now you can run this container with your host directory mounted:

docker run --rm -it -v ~/project/docker-test:/home/developer/project dev-container bash

Inside the container, try to access the files:

ls -la ~/project
cat ~/project/testfile.txt

This works because:

1. The container user has the same UID as your host user
2. The file permissions allow access to that UID
3. The mounted directory maintains the same ownership and permissions

Using Docker Compose for Consistent Volume Setups

For more complex applications, Docker Compose can help maintain consistent volume configurations. Let's create a simple Docker Compose file:

mkdir -p ~/project/compose-test
cat > ~/project/compose-test/docker-compose.yml << EOF
version: '3'

services:
  app:
    image: ubuntu
    user: "\${UID}:\${GID}"
    volumes:
      - ./data:/app/data
    command: ["bash", "-c", "echo 'Running as user \$(id -u):\$(id -g)' > /app/data/output.txt && cat /app/data/output.txt"]

volumes:
  app_data:
EOF

## Create the data directory
mkdir -p ~/project/compose-test/data

## Run with your user ID
cd ~/project/compose-test
UID=$(id -u) GID=$(id -g) docker compose up

This approach:

1. Uses environment variables to pass your user ID and group ID to Docker Compose
2. Sets the container to run with your user ID
3. Mounts a local directory that is accessible by your user

After running, check the output file:

cat ~/project/compose-test/data/output.txt

You should see:

Running as user 1000:1000

This confirms that the container ran with your user ID and had the appropriate permissions to write to the mounted volume.

Summary

In this lab, you learned how to identify, troubleshoot, and resolve the 'permission denied' error when mounting volumes in Docker. The key points covered include:

• Understanding how Docker volumes work and their benefits for data persistence
• Identifying permission issues when mounting host directories as volumes
• Resolving permission errors using multiple techniques:
  • Adjusting file permissions on the host
  • Running containers with specific user IDs
  • Managing ownership of files and directories
• Best practices for setting up Docker volumes with proper permissions:
  • Using named volumes for better permission handling
  • Creating consistent development environments with matching user IDs
  • Using Docker Compose for complex volume setups

These skills are essential for working with Docker in real-world scenarios where data persistence and proper permission management are critical. By understanding how Docker handles permissions for mounted volumes, you can avoid common issues and create more robust containerized applications.