Manually creating custom images as shown in the previous section of this chapter is very helpful when doing exploration, creating prototypes, or making feasibility studies. But it has a serious drawback: it is a manual process and thus is not repeatable or scalable. It is also as error-prone as any task executed manually by humans. There must be a better way.
This is where the so-called Dockerfile comes into play. The Dockerfile is a text file that is usually literally called Dockerfile. It contains instructions on how to build a custom container image. It is a declarative way of building images.
In computer science, in general and with Docker specifically, one often uses a declarative way of defining a task. One describes the expected outcome and lets the system figure out how to achieve this goal, rather than giving step-by-step instructions to the system on how to achieve this desired outcome. The latter is the imperative approach.
Let's look at a sample Dockerfile:
FROM python:2.7
RUN mkdir -p /app
WORKDIR /app
COPY ./requirements.txt /app/
RUN pip install -r requirements.txt
CMD ["python", "main.py"]
This is a Dockerfile as it is used to containerize a Python 2.7 application. As we can see, the file has six lines, each starting with a keyword such as FROM, RUN, or COPY. It is a convention to write the keywords in all caps, but that is not a must.
Each line of the Dockerfile results in a layer in the resulting image. In the following image, the image is drawn upside down compared to the previous illustrations in this chapter, showing an image as a stack of layers. Here, the base layer is shown on top. Don't let yourself be confused by this. In reality, the base layer is always the lowest layer in the stack:
Now let's look at the individual keywords in more detail.