When writing a Dockerfile to create a Docker image, here are the five essential steps that every DevOps engineer typically follows to build efficient, portable, and reliable container images.
Also read: What is Real Nation-Building and How to Live Like the Heroes Who Built India
Contents
What is a Docker Image?
A Docker image is a layered abstraction packed into a single file, which can be shipped anywhere and run as a container on any system with Docker runtime. Images contain everything needed to run the application, including the OS, dependencies, and the application code itself.
Step 1: Set the Base Image
Select a suitable base image that functions as your container’s operating system. It can be something lightweight like Alpine Linux or a language-specific image, such as "python:3.8-alpine". Using minimal base images helps keep your container small and secure.
Note: It is common to set the working directory during this stage with the "WORKDIR" instruction to indicate where all subsequent commands will be executed inside the container.
Step 2: Copy the Dependency Files
Copy only the dependency descriptor files first (e.g., "pom.xml" for Java, "package.json" for Node.js, or "requirements.txt" for Python). This allows Docker to leverage caching by only reinstalling dependencies if these files change.
Step 3: Copy the Source Code
Once dependencies are installed, copy your application source code into the container. This keeps your build incremental and efficient.
Step 4: Install Dependencies
Run appropriate commands to download and install the dependencies listed in the files copied earlier. For example, "npm install" for Node.js or "pip install -r requirements.txt" for Python.
Step 5: Run the Application
Finally, define the command to run your application like "npm start" or "python app.py". This is the entry point when the container starts.
Example for a Node.js application: Dockerfile
FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build
CMD ["npm", "start"]
Explanation
-
Using
"node:18-alpine"keeps the base image small and secure due to Alpine Linux’s minimal footprint. -
Copying
"package*.json"separately ensures Docker cache efficiency; dependencies are only reinstalled when these files change. -
Running
"npm ci"installs dependencies exactly as described in the lockfile, resulting in clean and repeatable builds. -
Copying the remainder of the source code afterward supports incremental builds and faster rebuilds when code changes.
-
Running
"npm run build"compiles/transpiles/bundles the application before runtime, producing optimized production-ready files. -
The
"CMD"The instruction defines how the app will run inside the container once it starts.
Pro Tip
Always check with your development team if they have additional build steps or environment variables needed to ensure smooth Docker builds and deployments.
For more lightweight images, we can build using multi-stage builds. I will cover this in my upcoming blogs.
