Building images with docker build - Commands & Configuration

Introduction

Building a Docker image packages your app and its environment into a single file. This lets you run your app anywhere without setup problems.

When you want to package your web app with all its software so it runs the same on any computer.

When you need to share your app with teammates or deploy it to a server easily.

When you want to test your app in a clean environment without installing anything on your machine.

When you want to update your app and create a new version to deploy.

When you want to automate app setup in continuous integration pipelines.

Config File - Dockerfile

Dockerfile

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
CMD ["python", "app.py"]

FROM sets the base image with Python 3.11 slim version.

WORKDIR sets the working directory inside the image.

COPY requirements.txt . copies the file with dependencies.

RUN pip install installs the dependencies.

COPY . . copies all app files into the image.

CMD defines the command to run the app.

Commands

This command builds a Docker image named 'my-app-image' with tag '1.0' using the Dockerfile in the current directory. It packages your app and its environment.

Terminal

docker build -t my-app-image:1.0 .

Expected Output

Sending build context to Docker daemon  4.096kB

Step 1/6 : FROM python:3.11-slim

 ---> 123abc456def

Step 2/6 : WORKDIR /app

 ---> Using cache

 ---> 789def012abc

Step 3/6 : COPY requirements.txt .

 ---> Using cache

 ---> 345ghi678jkl

Step 4/6 : RUN pip install --no-cache-dir -r requirements.txt

 ---> Running in abc123def456

Collecting flask

Installing collected packages: flask

Successfully installed flask-2.3.2

Removing intermediate container abc123def456

 ---> 901mno234pqr

Step 5/6 : COPY . .

 ---> 567stu890vwx

Step 6/6 : CMD ["python", "app.py"]

 ---> Running in def789ghi012

Removing intermediate container def789ghi012

 ---> 345yz123abc

Successfully built 345yz123abc

Successfully tagged my-app-image:1.0

→

-t - Assigns a name and tag to the image for easy reference

Lists all Docker images on your machine so you can verify your new image was created.

Terminal

docker images

Expected Output

REPOSITORY      TAG       IMAGE ID       CREATED          SIZE

my-app-image    1.0       345yz123abc    10 seconds ago   120MB

Runs a container from your newly built image to check that your app starts correctly. The --rm flag removes the container after it stops.

Terminal

docker run --rm my-app-image:1.0

Expected Output

 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

→

--rm - Automatically removes the container after it stops to keep your system clean

Key Concept

If you remember nothing else from this pattern, remember: docker build creates a reusable image from your app and environment using a Dockerfile.

Common Mistakes

Not running the docker build command in the directory containing the Dockerfile.

Docker cannot find the Dockerfile and fails to build the image.

Always run docker build in the folder where your Dockerfile is located or specify the correct path.

Forgetting to tag the image with -t, resulting in an image with a random ID only.

It becomes hard to find and run the image later without a meaningful name.

Use the -t flag to give your image a clear name and version tag.

Not copying all necessary files into the image, causing runtime errors.

Your app may fail because required files or dependencies are missing inside the image.

Use COPY commands in the Dockerfile to include all needed files.

Summary

Write a Dockerfile to define your app environment and files.

Use docker build -t name:tag . to create an image from the Dockerfile.

Check your image with docker images and run it with docker run.