Hi! I have docker builds that are dependent on one another and currently am using this syntax.

ARG BUILD_IMAGE=//apps/ingress-nginx/build:docker-amd64
ARG BASE_IMAGE=//base-images/alpine:docker-amd64

I’m trying to make this support arm64 as well and we dont have the capability for multi arch manifests (yet) so I need to explicitly specify the arch. I tried this and it doesnt work

ARG BUILD_IMAGE=//apps/ingress-nginx/build:docker-${TARGET_ARCH}
ARG BASE_IMAGE=//base-images/alpine:docker-${TARGET_ARCH}

Any guidance on how to effectively do this?

heya. Sorry for the trouble. When you say "it doesn't work", what does that mean? Are you seeing errors or unexpected behaviour?

Yep- the pants builds errors out because it cannot interpolate the

TARGET_ARCH

argument into the build or base images Heres an example I just ran:

ARG TARGETARCH
ARG BASE_IMAGE=//base-images/alpine:docker-${TARGETARCH}

results in

f"{arg_name}={address_to_built_image_tag[addr]}"
KeyError: Address(base-images/alpine:docker-${TARGETARCH})

Ah, failing with a

KeyError

is definitely a bug. Could you file an issue? Thanks. In terms of making this work… where does

TARGET_ARCH

get set? Is it provided as an environment variable to the pants invocation?

It should be inferred by buildkit- I did however pass in it as as var to the pants invocation and got the same result. Ill file a bug

Ah, I see. If you're willing to pull the instructions into the

BUILD

file rather than as a separate

Dockerfile

(using https://www.pantsbuild.org/stable/reference/targets/docker_image#instructions), then one can likely format the spec earlier so that Pants' dep inference gets the fully formatted value, rather than rely on docker's env-var substitution that only happens when actually building: • If you're willing to provide an env-var (not necessarily

TARGETARCH

) externally, then

instructions=[...., f"ARG BASE_IMAGE=//base-images/alpine:docker-{env('YOUR_VARIABLE_NAME_HERE')}", ....]

(

env

also accepts an optional default value for if the var isn't set: https://www.pantsbuild.org/stable/docs/using-pants/key-concepts/targets-and-build-files#environment-variables) • Alternatively, one could have duplicate

docker_image

targets via a loop (or a python function) e.g.

for arch in ...:
    docker_image(..., instructions=[..., f"ARG BASE_IMAGE=//base-images/alpine:docker-{arch}", ...], ...)