If you're starting with Docker, understanding Dockerfile instructions is very important.

A Dockerfile is simply a blueprint used to build Docker images.

Let’s break down the most commonly used Dockerfile components 👇

FROM - Specify the base image.

RUN - Execute a command in the image during build.

CMD ["executable","param1","param2"] - Specify the default command to run in a container.

ENTRYPOINT ["executable", "param1", "param2"] - Set the entry point for a container.

WORKDIR /path - Set the working directory for instructions.

COPY - Copy files or directories from host to image.

ADD - Copy files or directories from host to image with URL support and automatic unpacking of compressed files.

ENV - Set environment variables in the image.

EXPOSE - Inform Docker that the container listens on the specified network ports at runtime.

VOLUME ["/data"] - Create a mount point with the specified path and mark it as holding externally mounted volumes from native host or other containers.

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1️⃣ FROM – Base Image

Specifies the base image for your Docker image.

Every Dockerfile must start with FROM (except multi-stage builds where it may appear multiple times).

FROM ubuntu:22.04

📌 Example:

FROM node:18

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2️⃣ RUN – Execute Commands During Build

Used to execute commands while building the image.

RUN apt update && apt install -y nginx

👉 This runs only during the image build process.

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3️⃣ CMD – Default Command

Specifies the default command to run when a container starts.

CMD ["executable", "param1", "param2"]

📌 Example:

CMD ["node", "app.js"]

⚡ Only one CMD is allowed. If multiple are provided, the last one will be used.

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4️⃣ ENTRYPOINT – Main Container Process

Sets the main command that always runs when the container starts.

ENTRYPOINT ["executable", "param1", "param2"]

📌 Example:

ENTRYPOINT ["python3"]

🆚 Difference:

• CMD can be overridden.

• ENTRYPOINT is harder to override.

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5️⃣ WORKDIR – Set Working Directory

Sets the working directory for all following instructions.

WORKDIR /app

📌 Automatically creates the directory if it doesn't exist.

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6️⃣ COPY – Copy Files

Copies files from your local system into the Docker image.

COPY source destination

📌 Example:

COPY . /app

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7️⃣ ADD – Copy with Extra Features

Similar to COPY but:

• Supports URLs

• Automatically extracts compressed files

ADD file.tar.gz /app

⚠️ Best Practice: Prefer COPY unless you specifically need ADD features.

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8️⃣ ENV – Environment Variables

Sets environment variables inside the image.

ENV APP_ENV=production

📌 Used for configuration settings.

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9️⃣ EXPOSE – Declare Container Port

Informs Docker which port the container listens on.

EXPOSE 8080

⚠️ Note: It does NOT publish the port.

You still need -p option while running container.

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🔟 VOLUME – Persistent Storage

Creates a mount point for external storage.

VOLUME ["/data"]

📌 Used for:

• Databases

• Logs

• Persistent files

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🔥 Simple Example Dockerfile

FROM node:18

WORKDIR /app

COPY package*.json ./

RUN npm install

COPY . .

EXPOSE 3000

CMD ["node", "app.js"]