Dockerfile Basics

A Dockerfile is a text file that contains the “recipe” for building a Docker image. It’s how you tell Docker exactly what your application needs to run.

Key Dockerfile Instructions

1. FROM (Starting Point)

Every Dockerfile must start with FROM. It specifies the base image you want to build on top of.

• Example: FROM node:18 (Starts with a pre-configured Node.js environment).

2. WORKDIR (The Main Folder)

Sets the “current directory” inside the container for any subsequent commands.

• Example: WORKDIR /app

3. COPY (Moving Files In)

Copies files from your local computer into the Docker image.

• Example: COPY . . (Copies everything in the current folder).

4. RUN (Executing Commands)

Runs a command while the image is being built (like installing a library).

• Example: RUN npm install

5. EXPOSE (Opening a Port)

Tells Docker that the container will listen on a specific port at runtime.

• Example: EXPOSE 3000

6. CMD (The Startup Command)

The command that runs when the container is finally started. A Dockerfile can only have one CMD.

• Example: CMD ["npm", "start"]

Example Dockerfile for a Node.js App

# Step 1: Start with Node.js
FROM node:18

# Step 2: Set the working directory
WORKDIR /app

# Step 3: Copy our package files first (for efficiency!)
COPY package*.json ./

# Step 4: Install our dependencies
RUN npm install

# Step 5: Copy the rest of our code
COPY . .

# Step 6: Open the port our app uses
EXPOSE 3000

# Step 7: Start the application
CMD ["npm", "start"]

Architecture Diagram (The Build Process)

       [ Dockerfile ] (The Recipe)
               ↓ (docker build)
       [ Docker Image ] (The Finished Cake)
               ↓ (docker run)
       [ Docker Container ] (Eating the Cake)

How Junior Developers Encounter This

• “We need to add a library to the container”: (Updating the RUN command).
• “Why isn’t the new code showing up?”: (Forgetting to re-run the COPY command during build).
• “I need to change the base image”: (Updating the FROM command).

Common Beginner Mistakes

• Not Using WORKDIR: If you don’t set a workdir, your files will be scattered in the root directory, which is messy and dangerous.
• Putting COPY . . at the Top: This makes the build slow! Always copy your dependency files (package.json, requirements.txt) first and run your install command before copying the rest of your code. (This is called “Layer Caching”).
• Forgetting the CMD: If you don’t provide a CMD, your container will start and then immediately stop because it has nothing to do.

Next Lesson

Now that you’ve written your first recipe, let’s learn how to bake the cake:

➡ Building Images