Set condition based on CPU-Arch in Dockerfile

Question

I need to download and install a package directly from GitHub and I need to install some libraries I need for a build from source through pip down the line.

For that I use:

RUN apt-get update && apt-get install -y libavformat-dev libavdevice-dev libavfilter-dev libswscale-dev

and

RUN wget https://github.com/mozilla/geckodriver/releases/download/v0.30.0/geckodriver-v0.30.0-linux64.tar.gz \
&& tar -xf geckodriver-v0.30.0-linux64.tar.gz \
&& mv geckodriver /usr/local/bin/ \
&& rm geckodriver-v0.30.0-linux64.tar.gz

I want to build for different platforms with buildx: docker buildx build --platform linux/amd64,linux/arm64,linux/arm/v7 .

On amd64 I do not need to install the av libraries, as pip won't need to build anything, because wheels are provided. On arm64 and arm/v7 I need to install the libraries, and I need to download, extract and copy a different geckodriver package.

Is there a way to specify conditional statements based on CPU architecture?

Revile

Answer 1

As Daniel mentioned, Docker provides predefined environmental variables that are accessible at container build time.

We don't need to do anything Docker specific with these however, we can simply use them in a bash script condition:

FROM python:3-bullseye

RUN apt-get update && apt-get install -y ffmpeg firefox-esr npm
ARG TARGETARCH
RUN if [ $TARGETARCH = "arm64" ]; then \
        apt-get install -y libavformat-dev libavdevice-dev python3-av \
    ; fi
RUN npm install -g webdriver-manager
RUN webdriver-manager update --gecko

Note the necessary specification of ARG TARGETARCH to make that environmental variable accessible by dispatched processes at build time.

Revile

Answer 2

Following this docker blog post you have some predefined ARG variables like BUILDPLATFORM that you can use to build your image.

Therefore, you can try something like this:

FROM alpine as arm64
RUN echo "arm64" > /version.txt

FROM alpine as amd64
RUN echo "amd64" > /version.txt

FROM $TARGETARCH as common
RUN date >> /version.txt

CMD ["cat", "/version.txt"]

And use docker buildx to compile the image for multiple platforms:

$ docker buildx build --platform linux/amd64 -t test .
$ docker run --rm test
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
amd64
Thu May 25 07:54:34 UTC 2023

$ docker buildx build --platform linux/arm64 -t test .
$ docker run --rm test
arm64
Thu May 25 07:53:09 UTC 2023

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