Docker doesn't download recommended packages - linux

Question

I am using docker for a Python application.
FROM python:3.5-slim
WORKDIR /abc
ADD . /abc
RUN apt-get update && \
apt-get install -y --no-install-recommends \
curl \
gcc \
python3-dev \
musl-dev \
&& \
pip install -r requirements.txt &&\
apt-get clean && \
rm -rf /var/lib/apt/lists/* &&\
apt-get purge -y --auto-remove gcc
So whenever I am running the docker build command it first runs the apt-get update command there.
With update command, it's also downloading many recommended packages and taking long build time.
How can I stop Ubuntu from installing recommended packages and build docker faster?
Note: In the Dockerfile, apt-get --no-install-recommends update is not working; it's still downloading packages.

Answer 1

apt-get update should not install anything. The only thing apt-get update should do is update the local description of what packages are available. That does not download those packages though -- it just downloads the updated descriptions. That can take a while.
apt-get install will of course install packages. In order to install those packages, it needs to download them. Using --no-install-recommends tells apt-get to not install "recommended packages". For example, if you install vim, there are many plugins that are also recommended and provided as separate packages. With that switch, those vim plugins will not be installed. Of course, installing the packages you selected can also take a while.
What you're doing, using && \ is to put all of that into a single docker command. So every time you rebuild your image, you will have to do that every time because the list of packages changes every day, sometimes even multiple times per day.
Try moving pip install -r requirements.txt to its own RUN command after you've run apt-get stuff. If that then does what you want, then I suggest reading and learning more about how Docker works under the hood. In particular, it's important to understand how each single command adds a new layer and how any dynamic information in a single layer can cause long build times because the layer will frequently change with large amounts of changes.
Additionally, you might want to move ADD . /abc to after the RUN commands. Any changes you've made to the files being added (source code, I assume) will invalidate the layer which represents the apt-get command that has been executed. Since it's been invalidated, it will need to be rebuilt. If you're actively working on and developing those projects, that can easily cause apt-get to be executed every time you build your image.
There are plenty of resources you can search for which discuss how to optimize your time when using Docker. I won't recommend any specific one and will leave it to you for learning.