Welcome to Day 17 of 21 Days of Docker. The topic for today is the Building Effective Docker Image.
What are containers layers?
On Day 8, I talked about Docker Images and layers.
Image is build of a bunch of image layers and when we boot the container using this image, container adds a read-write layer on the top of it.
Now at this stage, the question you need to ask yourself, why do I care how many layers I have?
- More layers mean a larger image. The larger the image, the longer it takes to both build, push and pull from the registry.
- Smaller images mean faster builds and deploys.
How can I reduce my layers?
- Use shared base images where possible.
- Limit the data written to the container layer.
- Chain RUN statements
- Prevent cache misses at build for as long as possible.
Other steps we can do, to build an optimized image
Choosing the right base image
centos latest 9f38484d220f 7 months ago 202MB
python 3.7.3-alpine 2caaa0e9feab 3 months ago 87.2MB
- In this case, if I am building a Python application, I don’t need entire centos operating, I just need a minimal base OS(which is provided by alpine), with all the Python binaries installed on the top of it.
FROM ubuntu:latest --> FROM python:3.7.3-alpine LABEL maintainer="firstname.lastname@example.org" RUN apt-get update -y && \-->we don't need this as Python image already include it apt-get install -y python-pip python-dev COPY ./requirements.txt /app/requirements.txt WORKDIR /app RUN pip install -r requirements.txt COPY . /app ENTRYPOINT [ "python" ] CMD [ "app.py" ]
NOTE: There might be cases where you need a full base OS
- Ease of development
Use of Scratch
- It’s a special empty Dockerfile.
- Use this to build your own base images.
- OR build a minimal image that run a binary and nothing else.
FROM scratch COPY hello / CMD ["/hello"]
Use Multi-stage Build as I mentioned on Day7
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