Understanding "Mounts Denied" in Docker on macOS

Introduction

Docker is a powerful containerization tool that enables developers to create, deploy, and run applications in isolated environments called containers. While it's widely used across different platforms, users on macOS often encounter a unique error message: "Mounts denied: The paths...are not shared from OS X and are not known to Docker." Understanding why this happens and how to resolve it is vital for any developer working with Docker on macOS.

Technical Background

Docker on macOS

Docker involves two primary components:

• Docker Daemon: A server that manages all Docker objects like images, containers, and networks.
• Docker CLI: A command-line interface for users to interact with Docker.

Unlike Linux, where Docker runs natively, macOS uses a lightweight virtual machine to host the Docker Daemon. This is primarily due to the differences in kernel architecture.

Virtual File System

When mounting directories between your macOS host machine and Docker containers, the file paths need to be shared with the Docker virtual machine (usually using HyperKit or the newer Lima VM). If paths are not shared, the error "Mounts denied" appears.

Causes of "Mounts Denied" Error

1. Unshared Paths:
   • Only specific paths on your macOS filesystem are shared with Docker by default. This includes `/Users`, `/Volumes`, `/private`, and `/tmp`.
2. Configuration Issues:
   • If paths are outside these shared locations and not explicitly configured to be shared, Docker will not recognize them.
3. Permission Constraints:
   • Insufficient permissions may prevent Docker from accessing specific paths.

Resolving "Mounts Denied"

Step-by-Step Solution

1. Check Docker Preferences:
   • Open Docker Desktop.
   • Go to Settings > Resources > File Sharing.
   • Ensure your directory appears in this list. If not, add it manually.
2. Modify `docker-compose.yaml`:
   • Ensure the path you want to mount is specified correctly and falls under shared directories.
     • /Users/you/shared-folder:/app
   • Check and adjust permissions for the directories:
   • After making changes, restart Docker to apply them.
     • /Users/you/Development:/app

• Security Implications:
  • Be cautious about sharing sensitive directories because once shared, these can potentially become accessible from any running container.
• Performance Overheads:
  • Accessing files from volumes can be slower compared to native Docker on Linux due to macOS's need to virtualize the Docker environment.