Mastering Dockerfile: Key Instructions Explained in Simple Terms

by Mr. IT-man 03 February 2025 2

If you're working with Docker, mastering Dockerfiles is essential! A Dockerfile is a script that...

If you're working with Docker, mastering Dockerfiles is essential! ADockerfileis a script that defines how your Docker image is built. In this post, I'll break downessential Dockerfile instructionswith examples, comparisons, and best practices. ????

1️⃣ FROM – Define Base Image

Every Dockerfilemuststart with FROM , which specifies the base image.

Example:

FROM node:18

Best Practice:Useslimoralpineversions for smaller images:

FROM node:18-slim

✅Reduces image sizeand improves security.

2️⃣ WORKDIR – Set Working Directory

Defines where the app runs inside the container.

Example:

WORKDIR /app

Why use it?

✔️ Avoids cd commands.

✔️ Keeps the directory structure clean.

3️⃣ COPY vs ADD – Copying Files

Both copy files into the container, butADDcan also extract archives.

Instruction	Function	Extracts `.tar.gz`?	Supports URLs?
COPY	Copies files/folders	❌ No	❌ No
ADD	Copies + extracts files	✅ Yes	✅ Yes

Example:

COPY package.json .
ADD my-archive.tar.gz /data/

✅Best Practice:Use COPY unless youneed extraction.

4️⃣ RUN – Execute Commands During Build

Used to install dependencies or configure the container.

Example:

RUN apt-get update && apt-get install -y curl

✅Best Practice:Use && to reduce layers.

5️⃣ CMD vs ENTRYPOINT – What’s the Difference?

These define how thecontainer starts, but they work differently.

Instruction	Purpose	Can Be Overridden?
CMD	Default command	✅ Yes
ENTRYPOINT	Fixed command	❌ No

CMD Example:

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

ENTRYPOINT Example:

ENTRYPOINT ["java", "-jar", "app.jar"]

✅Use CMDif users might override the command.

✅Use ENTRYPOINTfor fixed commands like Docker CLI tools.

6️⃣ EXPOSE – Define Port

Tells Docker which port the app will use.

Example:

EXPOSE 3000

Note:EXPOSEdoes notactually open the port! You still need -p when running the container.

docker run -p 3000:3000 my-app

7️⃣ ENV – Set Environment Variables

Used to defineconfiguration variables.

Example:

ENV NODE_ENV=production

Best Practice:Use .env files for secrets instead of hardcoding values.

8️⃣ VOLUME – Persistent Storage

Ensures data isnot lostwhen the container stops.

Example:

VOLUME /data

✅Useful for:Databases, logs, and file storage.

9️⃣ ARG vs ENV – Build vs Runtime Variables

Instruction	Purpose	Available at Runtime?
ARG	Temporary build-time variable	❌ No
ENV	Runtime configuration	✅ Yes

Example:

ARG BUILD_VERSION=1.0
ENV APP_ENV=production

✅Use ARGfor build-time secrets.

✅Use ENVfor runtime configs.

Optimized Dockerfile Example

FROM node:18-slim
WORKDIR /app
COPY package.json .
RUN npm install --production
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

✅Best Practices Applied:

Uses aslim base image.
Sets aworking directory.
Minimizes layersin RUN .
Avoids unnecessary files.
UsesCMD for flexibility.

Summary

Instruction	Purpose
FROM	Defines the base image
WORKDIR	Sets working directory
COPY vs ADD	Copies files (ADD also extracts)
RUN	Executes build commands
CMD vs ENTRYPOINT	Defines how the container runs
EXPOSE	Declares port usage
ENV	Sets environment variables
ARG	Temporary build-time variables
VOLUME	Persistent storage