How can I get the Docker target platform inside the Build Environment / Dockerfile

Question

I'm doing multi-platform builds with buildx and would like to know if there is a way to determine what platform is currently being build so I can avoid certain steps for certain environments.

For example, something like...

# No DOSEMU outside amd64
RUN if [ "$DOCKER_PLATFORM" = "amd64" ]; then \
      apt install dosemu \
    fi

Answer 1

This is typically done with a script, not the Dockerfile. The script (perhaps in bash or python) can run at build time to determine what environment it is in, and act accordingly. This can be included in the RUN command, but it risks becoming quite complex and difficult to read in the Dockerfile. Furthermore, each additional RUN or CMD entry in the Dockerfile adds a layer to the image, which is considered bad practice.

This script can instead take the arguments BUILDPLATFORM and TARGETPLATFORM as suggested in the buildx docs, which would make more declarative. Instead of discovering the target, you would declare it (and write your script accordingly), so that the user understands exactly what happens without having to look inside the build.

Answer 2

One other option is to use TARGETPLATFORM as mentioned but with multistage Dockerfile.

If there's no complex logic apart from a few different steps (between targets), this allows to keep everything in the Dockerfile (and a single one).

FROM alpine:3.16 as main-linux-armv7
# do whatever is different on linux/arm/v7

FROM alpine:3.16 as main-linux-amd64
# do whatever should be for linux-amd64

FROM main-linux-amd64 as main-linux-arm64
# linux-arm64 is the same as linux-amd64 but every target needs to be defined

FROM main-${TARGETOS}-${TARGETARCH}${TARGETVARIANT}
# main / final stage

Answer 3

I would just have a base script that have the things in common for most of the platform like base_install.sh

Then I would have multiple dockerfiles that can use this base script, an example for your amd64 dockerfile DockerfileAMD64:

FROM mybase64image

RUN ./base_install.sh
RUN apt-get install specific64things -y

and you can build it with:

docker build -f DockerfileAMD64

That way you can version control these files without duplicating code between the dockerfiles.

Answer 4

The simplest answer will be to use TARGETPLATFORM variable which is available when you run docker buildx build ... command. Only thing to remember is that for any variable that is available you need to declare it using ARG.

So, a sample Dockerfile snippet can be:

ARG TARGETPLATFORM
RUN if [ "$TARGETPLATFORM" = "linux/arm64" ]; then \
        echo "ARM64 version"; \
    else \
        echo "AMD64 version"; \
    fi