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Building docker images

Simple Flask application

First we will learn to run simple Flask application in a container.

#!/usr/bin/env python

from flask import Flask
app = Flask(__name__)

@app.route('/hi')
def hello_world():
    return 'Hello World!'
    
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

Here are the steps that we need to make to achieve this:

  • install python and flask
  • get our script into the container
  • acess locally flask application served from inside the container

Assuming the hello.py script with our flask app is in ./app directory, we can mount it inside the container with -v flag.

$docker run -v $PWD/app:/app

We also need to expose port 5000 from the container to our host. We can do it with -p flag.

$docker run -p 5000 ubuntu:14.04

Let us add this all up:

$ docker run -it -v $PWD/app:/app/ -p 5000 ubuntu:14.04
root@11e2c4d5b385:/# apt-get update
root@11e2c4d5b385:/# apt-get install -y python python-pip
root@11e2c4d5b385:/# pip install flask
root@11e2c4d5b385:/# cd /app
root@11e2c4d5b385:app/# chmod +x hello.py
root@11e2c4d5b385:app/# ./hello.py

We want to see the flask application via a web browser on local machine. To do that we need to know:

  • on what ip adress we can connect to the vm in which the container is running?
  • which port have been bound to port 5000 on the container
$ docker-machine ip dev
192.168.99.100
$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS                     NAMES
11e2c4d5b385        ubuntu:14.04        "/bin/bash"         24 minutes ago      Up 24 minutes       0.0.0.0:32768->5000/tcp   sick_swirles

Remember our flask app serves hellos on at /hi root, so we should be access it at

http://192.168.99.100:32768/hi

Building from Dockerfile with docker build

This is great, but it's a lot of work and bookkeeping. We will learn how to document and script this process. We will also build an image based on the requirements so that we can quickly start it when ever we need to.

We will create a file called Dockerfile in which we tell docker how to build an image for us.

FROM ubuntu:14.04
RUN apt-get update
RUN apt-get install -y python python-pip
RUN pip install flask
ADD $PWD/app /app

WORKDIR /app
EXPOSE 5000
CMD python hello.py

Lets explain what is happening:

  • FROM defines the base image we want to use
  • RUN will run a command in shell inside the container
  • ADD will copy files from local into the container, the first path is relative to Dockerfile location, second is relative to root in the container
  • WORKDIR will set the working directory inside the container
  • EXPOSE will expose the port in the container to the outside
  • CMD is the default command when running the container

Now we are going to build this image.

$ docker build -t flask .

We are labelling the image flask by using -t flag. We tell docker to look for Docker file in current working directory . .

$ docker images
REPOSITORY              TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
flask                   latest              86b0a35f9fa6        6 seconds ago       358.4 MB

Now we can run it.

$ docker run -d -P flask

We bind the ports with -P and run the container as a daemon, in the background with -d.

$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                     NAMES
20f2420ef0f4        flask               "python /app/hello.py"   2 seconds ago       Up 1 seconds        0.0.0.0:32770->5000/tcp   stupefied_yonath

We see the flask app can be accessed on port 32770. That means we should see it at:

http://192.168.99.100:32770/hi