04 - Dockerfile Basics

Dockerfile basics

We often need custom images to ship our own apps. Dockerfile defines how to build them.

What is a Dockerfile?

A text file of instructions telling Docker how to build an image—automated, consistent, repeatable.

Syntax basics

Instructions are uppercase, executed in order; each creates a new layer.

Common instructions

FROM

FROM ubuntu:20.04
FROM alpine:3.14

LABEL

LABEL maintainer="your-email@example.com"
LABEL version="1.0"

RUN

RUN apt-get update && apt-get install -y \
    nginx \
    curl \
    && rm -rf /var/lib/apt/lists/*

Combine related commands to reduce layers.

COPY vs ADD

COPY app.js /app/
ADD app.tar.gz /app/

Prefer COPY unless you need ADD features (auto-extract/URL).

WORKDIR

WORKDIR /app

ENV

ENV NODE_ENV=production
ENV PORT=3000

EXPOSE

EXPOSE 80

Declares intent; still map ports with -p at runtime.

CMD vs ENTRYPOINT

CMD ["nginx", "-g", "daemon off;"]
ENTRYPOINT ["nginx", "-g", "daemon off;"]

• ENTRYPOINT is the fixed command; CMD supplies default args and can be overridden by docker run.

VOLUME

VOLUME ["/data"]

USER

USER nginx

Best practices

1) Multi-stage builds

# build stage
FROM golang:1.17 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp

# final image
FROM alpine:3.14
WORKDIR /app
COPY --from=builder /app/myapp .
CMD ["./myapp"]

2) Minimize layers

Merge related RUN steps.

3) .dockerignore

node_modules
.git
*.log

4) Pin versions, avoid latest

FROM node:14.17.0-alpine

5) Clean caches in the same RUN

RUN apt-get update && apt-get install -y \
    package1 \
    package2 \
    && rm -rf /var/lib/apt/lists/*

Example: Node.js app image

FROM node:14-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install --production
COPY . .
EXPOSE 3000
CMD ["node", "app.js"]

Build and run

Build

docker build -t myapp:1.0 .

Run

docker run -d -p 3000:3000 --name myapp-container myapp:1.0

Summary

You learned Dockerfile syntax, key instructions, and best practices. Next: Docker data management (volumes, bind mounts, tmpfs).