add docker/buildx compatible tags

[LuckyTurtleDev]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

Since buildx exist out of docker image it would ideal to be used in the Docker From step inside a builder, to build mutliplatform images.
For this you must choose an different image tag for each platform. Docker/Buildx allows you to use environments variables like TARGETPLATFORM inside the from step. The problem is that docker does not allow to use function here. So there is no way to map TARGETPLATFORM to the needed rust tool chain.

Adding additional tags like musl-linux/arm64, musl-linux/arm/v7", etc would allow to use it as builder:

From --platform=$BUILDPLATFORM ghcr.io/rust-cross/rust-musl-cross:musl-$TARGETPLATFORM as builder

Describe why this would be a good inclusion for cross

It allows to use cross to build multiplatforms docker images without using DiD.
Hit this issue at ankitects/anki#2798, so currently only linux/amd64 is supported.

Maybe an solution for #1104

[Emilgardis]

I'm not sure I understand this feature request, I understand the spirit, but not how it would be used.

Could you explain a bit further how this would be done and used?

I'll also explain abit, today it's possible to do

[target.armv7-unknown-linux-musl.image]
name = "ghcr.io/rust-cross/rust-musl-cross"
toolchain = ["linux/arm/v7=armv7-unknown-linux-musl"]

#1104 exists to track how we would generate multiarch images with the cross-compiled binary inside.

side note, I don't think musl-linux is a valid docker/oci os platform

[LuckyTurtleDev]

I should clearyfi this more.
I do want use the docker image, used by cross inside the From step of a Dockerfile for building a rust program.

FROM --platform=$BUILDPLATFORM ghcr.io/rust-cross/rust-musl-cross:aarch64-unknown-linux-musl AS builder
SHELL ["/bin/bash", "-uo", "pipefail", "-c"]

ARG version

RUN apt-get -y update && apt-get -y install protobuf-compiler musl musl-dev musl-tools
ENV PROTOC=/usr/bin/protoc
RUN echo "build version ${version}"
RUN cargo install --locked --git https://github.com/ankitects/anki.git --tag ${version} anki-sync-server
RUN ldd /usr/local/cargo/bin/anki-sync-server


FROM --platform=$BUILDPLATFORM scratch
ENV SYNC_BASE=/data
COPY --from=builder --chmod=0755 /usr/local/cargo/bin/anki-sync-server /
CMD ["./anki-sync-server"]

So I do not use the cross binary, which mean I can not use the meta data section of the cargo.toml.
The problem is that I must change the target in the from step, which make it difficult, to build multi platform image.
Docker does provide the option to use docker Args here. Which are created by buildx.
So you can modifiy the From step like this:

FROM --platform=$BUILDPLATFORM ghcr.io/rust-cross/rust-musl-cross:$TARGETPLATFORM AS builder

The problem is that TARGETPLATFORM platform does not match the name of the rust toolchain. Instead it has values like linux/amd64, linux/arm64, linux/arm/v7. Sadly docker does not allow to use a function here, also variables for previous build steps can not be used, so they is currently no way to map this values to the required toolchain.

My suggestion is to add additional linux/amd64, linux/arm64, linux/arm/v7, etc tags, so TARGETPLATFORM platform can be used directly.

[Emilgardis]

aha! I think then that you're on the wrong issue tracker, this is cross-rs, for the cross binary that uses it's own docker images and produces binaries on the host system via cross build --target <target>

you probably want to raise this on https://github.com/rust-cross/rust-musl-cross instead! :D

[LuckyTurtleDev]

Ups sorry for this.

Issue moved to rust-cross/rust-musl-cross#133