When building Docker images, you may want to include a file that is outside the current build context—such as one from a parent directory. However, Docker has strict rules about what can be accessed during a build.
In this blog post, you’ll learn:
- ❓ Why Docker doesn’t allow access to parent folders by default
- 🔄 Workarounds to include files from outside the build context
- 🧪 Practical examples
- 🔐 Best practices for security and portability
❗ Problem: Can’t Access Parent Directory Files
Let’s say your folder structure looks like this:
project/
├── Docker/
│ └── Dockerfile
├── config/
│ └── settings.json
You want to copy ../config/settings.json into your Docker image. You try this in your Dockerfile:
COPY ../config/settings.json /app/settings.json
But it fails with an error like:
COPY failed: Forbidden path outside the build context: ../config/settings.json
🔒 Why This Happens
Docker’s COPY and ADD instructions only work with files inside the build context. This is a security feature to prevent unintended access to files on your system during image builds.
✅ Solution: Change the Build Context
The most straightforward fix is to set the build context at a higher level using the docker build command.
✅ Example:
Instead of running this (from Docker/):
docker build -t my-app .
Run it from the project root:
docker build -f Docker/Dockerfile -t my-app .
Now your COPY will work:
COPY config/settings.json /app/settings.json
✅ Why this works: You’re telling Docker the build context is the full
project/directory, so it can accessconfig/.
📁 Alternative: Move Files Temporarily
If changing the build context is not an option, another workaround is to temporarily copy the file into the build directory before building:
Example Bash Script:
cp ../config/settings.json ./Docker/
docker build -t my-app ./Docker
rm ./Docker/settings.json
⚠️ Be sure to clean up after the build to avoid accidental file inclusion or confusion.
🧠 Tip: Use .dockerignore Wisely
If you change the context to the root directory, make sure to use a .dockerignore file to avoid sending unnecessary files to Docker.
Example .dockerignore:
*.log
node_modules
.git
.env
✅ This reduces build time and keeps your image clean.
🔐 Best Practices
| Tip | Why It Matters |
|---|---|
| Never copy sensitive files from outside | Prevents security risks |
| Keep Dockerfile in project root if possible | Simplifies builds |
Use .dockerignore to control context | Improves performance and security |
| Keep Docker contexts consistent in teams | Avoids “works on my machine” issues |
🧠 Summary
| Goal | Recommended Approach |
|---|---|
| Copy file from parent directory | Set build context to project root |
| Prevent extra files from copying | Use .dockerignore |
| Can’t change context | Copy files manually before build |
🏁 Final Thoughts
Docker doesn’t allow accessing files outside the build context for good reason: security and consistency. The most robust solution is to structure your project so that your Dockerfile has access to everything it needs—within the context.
With a properly configured build context and .dockerignore, you’ll avoid common errors and build more portable, secure images.
