FROM instruction for base image in Docker - Commands & Configuration

Introduction

When you build a Docker image, you start from a base image that provides the basic environment. The FROM instruction tells Docker which base image to use, so you don't have to build everything from scratch.

When you want to create a custom application environment starting from a standard Linux system like Ubuntu or Alpine.

When you need to build a container image that includes a specific programming language runtime like Python or Node.js.

When you want to base your image on an official database image like MySQL or Redis to add custom configurations.

When you want to create a multi-stage build to optimize the final image size by using multiple FROM instructions.

When you want to specify a particular version or tag of a base image to ensure consistency across deployments.

Config File - Dockerfile

Dockerfile

FROM python:3.11-slim

WORKDIR /app

COPY . /app

RUN pip install --no-cache-dir -r requirements.txt

CMD ["python", "app.py"]

The FROM line sets the base image to Python 3.11 slim version, which is a lightweight Python environment.

WORKDIR sets the working directory inside the container.

COPY copies your application files into the container.

RUN installs the Python dependencies.

CMD defines the command to run your app when the container starts.

Commands

This command builds a Docker image named 'my-python-app' using the Dockerfile in the current directory. It reads the FROM instruction to start from the Python 3.11 slim base image.

Terminal

docker build -t my-python-app .

Expected Output

Sending build context to Docker daemon  4.096kB

Step 1/6 : FROM python:3.11-slim

3.11-slim: Pulling from library/python

Digest: sha256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Status: Downloaded newer image for python:3.11-slim

 ---> abcdef123456

Step 2/6 : WORKDIR /app

 ---> Running in 123456abcdef

Removing intermediate container 123456abcdef

 ---> 7890abcd1234

Step 3/6 : COPY . /app

 ---> abc123def456

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

 ---> Running in def456abc123

Collecting flask

  Downloading Flask-2.3.2-py3-none-any.whl (100 kB)

Installing collected packages: flask

Successfully installed flask-2.3.2

Removing intermediate container def456abc123

 ---> 456def789abc

Step 5/6 : CMD ["python", "app.py"]

 ---> Running in 789abc456def

Removing intermediate container 789abc456def

 ---> 123abc456def

Successfully built 123abc456def

Successfully tagged my-python-app:latest

→

-t - Assigns a name and optionally a tag to the image

This command lists all Docker images on your system, so you can verify that 'my-python-app' was created successfully.

Terminal

docker images

Expected Output

REPOSITORY      TAG       IMAGE ID       CREATED          SIZE

my-python-app   latest    123abc456def   10 seconds ago   120MB

python          3.11-slim abcdef123456   2 days ago       115MB

This command runs a container from the 'my-python-app' image to test that your app starts correctly. The --rm flag removes the container after it stops.

Terminal

docker run --rm my-python-app

Expected Output

 * Serving Flask app 'app'

 * Debug mode: off

WARNING: This is a development server. Do not use it in a production deployment.

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

→

--rm - Automatically removes the container when it exits

Key Concept

If you remember nothing else from this pattern, remember: the FROM instruction sets the starting point for your Docker image, defining the base environment your app will run in.

Common Mistakes

Using FROM without specifying a valid image name or tag

Docker will fail to build because it cannot find the base image to start from.

Always specify a valid base image name and tag, for example, 'FROM python:3.11-slim'.

Using 'latest' tag implicitly by writing 'FROM python' without a tag

This can lead to unpredictable builds because the 'latest' tag can change over time.

Specify an explicit tag like 'FROM python:3.11-slim' to ensure consistent builds.

Trying to use multiple FROM instructions without understanding multi-stage builds

Only the last FROM stage is used for the final image unless multi-stage build syntax is properly used.

Learn and use multi-stage build syntax properly if you need multiple FROM instructions.

Summary

The FROM instruction sets the base image for your Docker build.

Use docker build to create an image starting from the base image.

Verify the image with docker images and run it with docker run.