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Creating a custom Docker image ensures consistent deployments and lets you include exactly the dependencies your Python Flask app needs. Whether you require specialized system libraries or a private base image, building your own container image is straightforward.

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1. Planning Your Dockerfile

Before writing any code, outline the manual steps you’d perform to deploy your Flask application:
  1. Select a base image (e.g., Ubuntu or python:3-slim).
  2. Update and install OS packages (apt-get update && apt-get install).
  3. Install Python dependencies (pip install).
  4. Copy application source code into the image.
  5. Configure environment variables and expose required ports.
  6. Define the container’s startup command.
The image is a guide on creating a Docker image, listing steps such as using Ubuntu, updating the apt repository, installing dependencies, and running a web server with Flask.
Using an official Python base image (for example, python:3.9-slim) can reduce image size and simplify dependency installation.

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2. Writing the Dockerfile

Create a file named Dockerfile at your project root and add the following contents: 
# 1. Base image
FROM ubuntu:20.04

# 2. OS-level dependencies
RUN apt-get update && \
    apt-get install -y python3 python3-pip && \
    rm -rf /var/lib/apt/lists/*

# 3. Install Python packages
RUN pip3 install --no-cache-dir flask flask-mysql

# 4. Copy application source
WORKDIR /opt/source-code
COPY . .

# 5. Environment and ports
ENV FLASK_APP=app.py
EXPOSE 5000

# 6. Start the Flask server
ENTRYPOINT ["flask", "run", "--host=0.0.0.0"]
InstructionPurposeExample
FROMSets the base imageFROM ubuntu:20.04
RUNExecutes commands in a new layerRUN pip3 install flask flask-mysql
WORKDIRSets working directory inside the containerWORKDIR /opt/source-code
COPYCopies files from host to containerCOPY . .
EXPOSEDocuments the port the container listens onEXPOSE 5000
ENTRYPOINTDefines the startup commandENTRYPOINT ["flask", "run", ...]

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3. Build, Tag, and Push Your Image

  1. Build the image locally and add a tag: 
    docker build -t your-dockerhub-username/flask-app:latest .
  2. Push to Docker Hub (replace with your repository): 
    docker push your-dockerhub-username/flask-app:latest

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4. Inspecting Image Layers with docker history

Each Dockerfile instruction creates a new image layer. To view these layers and their sizes, run: 
docker history your-dockerhub-username/flask-app:latest
The output lists layers in reverse order, showing the command and size of each layer.
If you frequently change application code but not OS dependencies, structure your Dockerfile so that COPY . . appears after installing system and Python packages. This maximizes cache reuse and speeds up rebuilds.

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5. Leveraging Build Cache for Faster Iteration

Docker caches successful build steps. On subsequent builds:
  • Steps unchanged since the last build use cached layers.
  • Only modified steps (and those that follow) are re-executed.
Example: 
$ docker build -t flask-app:latest .
...
Step 2/6 : RUN apt-get update && apt-get install -y python3 python3-pip
 ---> Using cache
...
Step 5/6 : COPY . .
 ---> 123abc456def      # Rebuilds only this layer and ENTRYPOINT step
By isolating frequently changing instructions toward the end of your Dockerfile, you accelerate development cycles.

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