The Dockerfile

The special file that does the work of Dockerizing any of our applications is called a Dockerfile.

The Dockerfile is basically a blueprint for how our Docker image and Docker containers should be built. It contains all the instructions for how to Dockerize our application.

Important Dockerfile Instructions

FROM — Base Image

FROM node

Every Docker image we want to create is based on a base image.

When we write FROM, after it we define our base image.

What is a Base Image?

Our Node.js application requires Node to be set up on the system to run it.

Similarly, when our application becomes a Docker image or container, there are specific dependencies that should already be set up inside that container.

For a Node.js application, the base image is the node image.

This is also what we mean by layering — our test-app image is built on top of the Node image, which itself may be built on top of another base image (like Debian).

Layer on layer on layer — this is how layering works in Docker images.

ENV — Environment Variables

ENV MONGO_DB_USERNAME=admin \
    MONGO_DB_PASSWORD=qwerty

ENV is used to define environment variables inside the Docker image.

These variables become available inside the container during runtime.

RUN — Execute Commands Inside the Container

RUN mkdir -p /home/app

RUN is used to execute commands while building the Docker image.

We can have multiple RUN commands in a Dockerfile — for every different command we want to execute.

Each RUN instruction creates a new layer in the image.

COPY — Copy Files From Host to Image

COPY . /home/app

This copies files from the host machine into the Docker image.

Format:

COPY <host-path> <container-path>

Example:

• Host path: . (current directory)
• Container path: /home/app

This copies all files and folders from our current directory into the /home/app folder inside the container.

CMD — Default Command to Run the Container

CMD ["node", "/home/app/server.js"]

We can have multiple RUN commands but only one CMD command.

CMD is the single command that gets executed once the container is fully set up — it is what starts our application inside the container.

Important Notes:

• CMD defines the default startup command
• It runs only when the container starts
• Written in JSON array format (list of strings)

Example format:

CMD ["command", "arg1", "arg2"]

EXPOSE — Expose a Port

EXPOSE 5050

EXPOSE is used to inform Docker that the container listens on a specific port.

Important:

• It does NOT publish the port automatically
• It acts as documentation inside the Dockerfile
• Actual port mapping is done using -p in docker run

Summary of Dockerfile Instructions

Key Idea

A Dockerfile is a step-by-step blueprint that tells Docker:

• What base environment to start from
• What dependencies to install
• What files to include
• What command to run when the container starts

This is how a simple Node.js application becomes a fully containerized Docker image.