6. Dockerfile Basics

What Is a Dockerfile?

A Dockerfile describes how to build an image. Create an empty practice directory with these two files.

index.html:

<h1>Hello from the Docker course</h1>

Dockerfile:

FROM nginx:alpine
COPY index.html /usr/share/nginx/html/index.html
EXPOSE 80

The base image already defines how nginx starts. This example inherits that behavior.

Run these commands from the directory containing both files:

docker build -t docker-course-site:1 .
docker run -d --name docker-course-site -p 127.0.0.1:8081:80 docker-course-site:1

Visit http://localhost:8081 and confirm the heading.

Instruction	Purpose
FROM	Select a base image or begin a build stage
WORKDIR	Set the working directory
COPY	Copy files into the image
RUN	Execute a command during the build
CMD	Supply the default runtime command or arguments
ENTRYPOINT	Define the container executable
EXPOSE	Document a container port

EXPOSE alone does not publish a port on the host.

docker stop docker-course-site
docker rm docker-course-site

Change the heading, rebuild with tag 2, and run the new image. Explain why changing a local file does not modify an already built image.

RUN executes while building an image. CMD and ENTRYPOINT determine what runs when a container starts.