Multi-stage Dockerization of Flask Application

Multi-stage Dockerization of Flask Application

Multi-stage Docker builds help make smaller and better Docker images. For a Flask app, you can use multi-stage builds to split the build area, where you have development tools and dependencies, from the runtime area, which has only what's needed to run the app.

Here's an example Dockerfile for a Flask application using multi-stage builds:

# Build Stage
FROM python:3.9 AS builder

# Set the working directory in the container
WORKDIR /app

# Copy the requirements file to the working directory
COPY requirements.txt .

# Install dependencies (including build tools if needed)
RUN pip install --upgrade pip && \
    pip wheel --no-cache-dir --no-deps --wheel-dir /usr/src/app/wheels -r requirements.txt

# Runtime Stage
FROM python:3.9-slim

# Set the working directory in the container
WORKDIR /app

# Copy the application code and the wheels from the builder stage
COPY --from=builder /usr/src/app/wheels /wheels
COPY . .

# Install the application dependencies from the wheels directory
RUN pip install --no-cache-dir --find-links=/wheels -r requirements.txt

# Expose the port the app runs on
EXPOSE 5000

# Define environment variables
ENV FLASK_APP=your_app_name
ENV FLASK_RUN_HOST=0.0.0.0

# Run the application
CMD ["flask", "run"]

Explanation:

  • The first stage (builder) installs dependencies and makes a wheelhouse (a folder with Python wheels) for the app dependencies. This stage uses the full Python image.

  • The second stage uses a slim Python image to make the final image smaller. It copies the app code and wheels from the builder stage, installs the dependencies from the wheels folder, and prepares the environment for the Flask app.

  • The EXPOSE instruction shows that the app will use port 5000, and the CMD instruction tells the command to run when the container starts.

To use this Dockerfile, make a file called Dockerfile in your Flask app folder and add the content above. Also, make a requirements.txt file with your app's dependencies listed.

Build the Docker image using the following command:

docker build -t your_image_name .

Replace your_image_name with the desired name for your Docker image.

Run the Docker container:

docker run -p 5000:5000 your_image_name

This is a simple example, and you might need to change it depending on your Flask app's structure and needs. Always test the Dockerized app to make sure it works properly.

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