🧾 Dockerfile Command Reference

Each command in a Dockerfile defines a specific instruction for how to build a Docker image. Here’s a detailed breakdown:

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1️⃣ FROM — Set the Base Image 🏗️

FROM node:20-alpine

✅ What it does:
Defines the base image your custom image will build on top of.

📝 Must be the first instruction in the Dockerfile (except ARG sometimes).

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2️⃣ WORKDIR — Set Working Directory 📁

WORKDIR /app

✅ What it does:
Sets the working directory inside the container where all subsequent commands (COPY, RUN, etc.) will execute.

📝 Automatically creates the directory if it doesn’t exist.

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3️⃣ COPY — Copy Files into the Image 📦

COPY package*.json ./
COPY . .

✅ What it does:
Copies files/directories from your host into the container filesystem.

💡 Use .dockerignore to exclude unnecessary files.

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4️⃣ RUN — Execute a Shell Command 🛠️

RUN npm install
RUN apk add --no-cache curl

✅ What it does:
Runs a command (like installing packages, cleaning up files) at build time, and creates a new layer.

💡 Combine multiple commands to reduce image size:

RUN apk add --no-cache curl && npm install

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5️⃣ CMD — Default Command to Run on Container Start 🏁

CMD ["npm", "start"]

✅ What it does:
Specifies the default command that gets run when the container starts.

💡 Only one CMD allowed — if multiple, only the last is used.

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6️⃣ EXPOSE — Document the App’s Port 🌐

EXPOSE 8000

✅ What it does:
Informs Docker that the app runs on a specific port — used for documentation and port binding.

💡 Does not actually publish the port (use -p in docker run).

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7️⃣ ENV — Set Environment Variables 🧪

ENV PORT=8000
ENV NODE_ENV=production

✅ What it does:
Defines environment variables inside the container, usable by the app or shell.

💡 Use process.env.PORT in Node.js.

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8️⃣ ARG — Build-Time Variables ⚙️

ARG NODE_VERSION=20-alpine
FROM node:${NODE_VERSION}

✅ What it does:
Defines variables that you can pass during docker build with --build-arg.

📝 Unlike ENV, ARG is not available during runtime.

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9️⃣ ENTRYPOINT — Hard-Coded Command 🚀

ENTRYPOINT ["node", "index.js"]

✅ What it does:
Defines the main executable — like CMD, but less override-able.

💡 Combine with CMD for arguments:

ENTRYPOINT ["npm"]
CMD ["start"]

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🔟 LABEL — Metadata About the Image 🏷️

LABEL maintainer="Darshan Vasani"
LABEL version="1.0"

✅ What it does:
Adds metadata like author, version, description to your image.

💡 Useful for scanning tools and image documentation.

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🔁 VOLUME — Define Persistent Storage Volumes 💾

VOLUME ["/app/data"]

✅ What it does:
Creates a mount point with a persistent volume at build/run time.

💡 Avoid putting app logic here; use it for data.

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🌎 USER — Switch to a Non-root User 🔐

RUN adduser -D myuser
USER myuser

✅ What it does:
Sets the user under which the container runs (for security best practices).

💡 Don’t run production apps as root.

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🧪 BONUS: Multi-Stage Build Example 🧠

# Stage 1: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .

# Stage 2: Runtime
FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app .
CMD ["npm", "start"]

✅ Why?

• Keep final image clean
• Remove dev dependencies
• Build-only stuff stays out of final image

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✅ Summary Table

Command	Purpose
FROM	Set base image
WORKDIR	Set working directory
COPY	Copy files into image
RUN	Execute shell commands at build time
CMD	Run command on container start
EXPOSE	Document listening port
ENV	Set environment variables
ARG	Pass build-time variables
ENTRYPOINT	Set main executable
LABEL	Add metadata to image
USER	Switch to non-root user
VOLUME	Declare persistent storage

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