🧾 Dockerfile Command Reference

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

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).

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.

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.

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 

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.

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).

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.

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.

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"] 

🔟 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.

🔁 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.

🌎 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.

🧪 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

✅ Summary Table