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dockerdockerfile

Use Dockerfile ARG to determine FROM platform

I want to pass in an optional argument to my docker file to determine the platform. If the platform is not available, I want docker to use the docker defaults.

Here is an example dockerfile that has a required argument:

ARG DOCKER_PLATFORM
FROM --platform=${DOCKER_PLATFORM} python:3.9-buster

RUN apt-get update
RUN apt-get install -y postgresql-client

...etc

Here is what I actually want to do:

ARG DOCKER_PLATFORM=
IF ${DOCKER_PLATFORM != ""
   FROM --platform=${DOCKER_PLATFORM} python:3.9-buster
ELSE:
   FROM python:3.9-buster

RUN apt-get update
RUN apt-get install -y postgresql-client

...etc

Can I have an conditional in docker? Is there anther way to remove this conditional?

Docker "FROM" docs that explain the --platform flag: https://docs.docker.com/engine/reference/builder/#from Note that $BUILDPLATFORM evaluates to an empty string, which is not an acceptable item to pass into --platform. I suppose that means I am not using BuildKit.

Solution

  • Docker provides several Automatic platform ARGs in the global scope, which include a BUILDPLATFORM and a TARGETPLATFORM. In principle you could use one of these as a default value

    ARG DOCKER_PLATFORM=$TARGETPLATFORM
FROM --platform=$DOCKER_PLATFORM python:3.9

    However, the default FROM --platform is the target platform, and the docker build --platform option sets this. So rather than using a custom ARG here, it may be enough for you to use the default platform

    # without a --platform option
FROM python:3.9

    and if you do need an alternate platform, specify it when you build

    docker build --platform=linux/amd64 ...

    I am not using BuildKit.

    BuildKit has been a non-experimental option since Docker 18.09; if it's not on by default, try setting an environment variable DOCKER_BUILDKIT=1 when you docker build.