How to create a docker file

[aravindm]

Dockerfile defines what goes on in the environment inside your container. Access to resources like networking interfaces and disk drives is virtualized inside this environment, which is isolated from the rest of your system, so you need to map ports to the outside world, and be specific about what files you want to “copy in” to that environment.

Create a file called Dockerfile, copy-and-paste the following content into that file, and save it.

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# Use an official Python runtime as a parent image
FROM python:2.7-slim

# Set the working directory to /app
WORKDIR /app

# Copy the current directory contents into the container at /app
ADD . /app

# Install any needed packages specified in requirements.txt
RUN pip install --trusted-host pypi.python.org -r requirements.txt

# Make port 80 available to the world outside this container
EXPOSE 80

# Define environment variable
ENV NAME World

# Run app.py when the container launches
CMD ["python", "app.py"]
This Dockerfile refers to a couple of files we haven’t created yet, namely app.py and requirements.txt. Let’s create those next.

Create two more files, requirements.txt and app.py, and put them in the same folder with the Dockerfile.

Dockerfile is built into an image, app.py and requirements.txt is present because of that Dockerfile’s ADD command, and the output from app.py is accessible over HTTP thanks to the EXPOSE command.

requirements.txt

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Flask
Redis

app.py

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from flask import Flask
from redis import Redis, RedisError
import os
import socket

# Connect to Redis
redis = Redis(host="redis", db=0, socket_connect_timeout=2, socket_timeout=2)

app = Flask(__name__)

@app.route("/")
def hello():
    try:
        visits = redis.incr("counter")
    except RedisError:
        visits = "<i>cannot connect to Redis, counter disabled</i>"

    html = "<h3>Hello {name}!</h3>" \
           "<b>Hostname:</b> {hostname}<br/>" \
           "<b>Visits:</b> {visits}"
    return html.format(name=os.getenv("NAME", "world"), hostname=socket.gethostname(), visits=visits)

if __name__ == "__main__":
    app.run(host='0.0.0.0', port=80)

Build the app

Make sure you are still at the top level of your new directory. Now run the build command. This creates a Docker image, which we’re going to tag using -t so it has a friendly name.

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docker build -t friendlyhello .
Where is your built image? It’s in your machine’s local Docker image registry:

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$ docker image ls

REPOSITORY            TAG                 IMAGE ID
friendlyhello         latest              326387cea398


Run the app

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docker run -p 4000:80 friendlyhello