COPY with Docker but with Exclusion

TLDR

To exclude files or directories when using the COPY instruction in Docker, leverage .dockerignore files. This ensures that unwanted files are not copied into your Docker image, optimizing build times and reducing image size.

---

The COPY instruction in Docker is used to copy files and directories from your local filesystem into a Docker image. However, there are scenarios where you might want to exclude certain files or directories from being copied. This guide will show you how to achieve this using .dockerignore files.

Step 1: Create a .dockerignore File

The .dockerignore file allows you to specify patterns for files and directories to exclude during the Docker build process. Create a .dockerignore file in the same directory as your Dockerfile.

Example .dockerignore File

node_modules
*.log
.env
temp/

Explanation

• node_modules: Excludes the node_modules directory.
• *.log: Excludes all .log files.
• .env: Excludes the .env file.
• temp/: Excludes the temp directory.

Step 2: Use the COPY Instruction

In your Dockerfile, use the COPY instruction to copy files into the image. The .dockerignore file will automatically exclude the specified files and directories.

Example Dockerfile

FROM node:18
WORKDIR /app
COPY . .
RUN npm install
CMD ["node", "app.js"]

Explanation

• COPY . .: Copies all files from the current directory into the /app directory in the image, excluding files specified in .dockerignore.
• RUN npm install: Installs dependencies.
• CMD ["node", "app.js"]: Starts the application.

Step 3: Build the Docker Image

Build the Docker image using the docker build command:

docker build -t my-app .

This command builds the image and excludes files specified in .dockerignore.

Advanced Options

Exclude Files Dynamically

You can dynamically exclude files by generating a .dockerignore file during the build process. For example:

echo "temp/" > .dockerignore
docker build -t my-app .

Multi-Stage Builds

Use multi-stage builds to copy only the necessary files into the final image:

FROM node:18 AS builder
WORKDIR /app
COPY . .
RUN npm install

FROM node:18
WORKDIR /app
COPY --from=builder /app/dist /app/dist
CMD ["node", "app/dist/app.js"]

Explanation

• The builder stage installs dependencies and builds the application.
• The final stage copies only the dist directory from the builder stage.

Best Practices

• Minimize Context: Use .dockerignore to reduce the build context size.
• Keep .dockerignore Updated: Regularly update .dockerignore to exclude unnecessary files.
• Test Builds: Test your Docker builds to ensure excluded files are not copied.

By following these steps, you can effectively use the COPY instruction in Docker with exclusion patterns, optimizing your Docker builds and improving image efficiency.