How to fix Mounts denied: The path /host/path is not shared from the host and is not known to Docker in Docker

First, identify the full path Docker is trying to mount. Check your docker command or docker-compose.yml:

# Check your docker run command
docker run -v /path/to/mount:/container/path myimage

# Or check docker-compose.yml for volume definitions
cat docker-compose.yml | grep -A 5 volumes

On macOS, if the path includes symlinks, resolve them:

realpath /path/to/your/directory

The resolved path is what Docker actually tries to access.

Navigate to Docker Desktop's file sharing configuration:

On macOS:
1. Click the Docker whale icon in the menu bar
2. Select "Preferences..." (or "Settings...")
3. Go to Resources > File Sharing

On Windows:
1. Right-click the Docker whale icon in the system tray
2. Select "Settings"
3. Go to Resources > File Sharing

Note: On Windows with WSL2 backend, file sharing is automatic for paths inside WSL distributions. The File Sharing tab only appears in Hyper-V mode.

Add your directory (or a parent directory) to Docker's file sharing list:

1. In the File Sharing settings, click the + button
2. Browse to and select the directory you need to mount
3. Click Apply & Restart

Tips:
- Share a parent directory (like /data) rather than individual subdirectories
- Avoid sharing your entire root filesystem for security reasons
- On macOS, sharing /usr/local may require additional permissions

After applying file sharing changes, Docker Desktop will restart:

1. Wait for the Docker whale icon to stop animating
2. Verify Docker is running:

docker info

3. Retry your original command:

docker run -v /your/shared/path:/container/path myimage

macOS uses symlinks in several system paths. Docker needs both the symlink and the real path shared.

For example, /var is symlinked to /private/var:

ls -la /var
# lrwxr-xr-x  1 root  wheel  11 Jan  1  2020 /var -> private/var

Solution: Add both paths to file sharing:
- /var/folders
- /private/var/folders

Or use the resolved path in your Docker command:

docker run -v "$(realpath /var/folders/xx/path):/container/path" myimage

If adding custom paths doesn't work or isn't possible, move your project to a default shared location:

On macOS (default shared paths):
- /Users (your home directory)
- /Volumes
- /private
- /tmp

On Windows:
- Your user profile folder (C:\Users\YourName)

Example:

# Move project to home directory
mv /custom/project ~/Projects/my-project
cd ~/Projects/my-project
docker-compose up

If you're experiencing persistent issues after a Docker Desktop update, try changing the file sharing backend:

1. Open Docker Desktop Preferences
2. Go to General
3. Find the file sharing implementation option
4. Try switching between:
- VirtioFS (faster, recommended)
- gRPC FUSE (more compatible)
- osxfs (Legacy) (older, slower but most compatible)
5. Click Apply & Restart

Some users report that switching from VirtioFS to gRPC FUSE resolves file sharing issues after updates.

If file sharing settings appear corrupted, reset them:

1. Open Docker Desktop Preferences
2. Go to Troubleshoot (or click the bug icon)
3. Click Reset to factory defaults

Warning: This removes all containers, images, volumes, and settings.

Alternatively, reset only the settings file:

On macOS:

rm ~/Library/Group\ Containers/group.com.docker/settings.json

On Windows:

Remove-Item "$env:APPDATA\Docker\settings.json"

Then restart Docker Desktop and reconfigure file sharing.