To write an application contained by Docker, one may start with an existing container that has the required environment. If the application is written in Python, a Python programming environment would make sense.
Docker's layered images allow us to take a Python image and layer the application on top of it. A Dockerfile describes what has to be done to create the new application container image. The following is a summary of the instructions on the Docker Get Started site:
- get a Python image
- specify the directory in the container where the application lives - its work directory
- add files to the container
- execute code, e.g. install other components or compile something
- map a network port from the container to the host
- set up some environment variables
- run the application
Corresponding Dockerfile:
FROM python:2.7-slim
WORKDIR /app
ADD . /app
RUN pip install --trusted-host pypi.python.org -r requirements.txt
EXPOSE 80
ENV NAME World
CMD ["python", "app.py"]The container is then created with the docker build command. Create a directory that contains the application and the Dockerfile, then
$ cd APPLICATION_DIRECTORY
$ docker build -t tag-for-my-app .
The tag will help finding the app on Docker hub or other repositories.
The build command adds the new container to the local repository, from where it can be executed:
docker run -p 4000:80 tag-for-my-app
The -p option maps the container's port 80 to the host's port 4000.