Allow multi-platform docker images

Arnaud Boutsrequested to merge
stoub/ci-templates:feat/multi-platform into main

To build a multi-platform docker image, we must provide the list of os/arch to target : --platform linux/amd64,linux/arm64

Providing arch and os in separated variables do not allow that without a hack :

TARGETOS: "linux"
TARGETARCH: "amd64,linux/arm64"

Removing TARGETOS and TARGETARCH should not impact related pipelines as these variables are injected as ARG during build phase from target platform.

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