[BUG] Multiarch images are indeed all x86_64

[iurly]

Describe the bug
All images (arm, s390x, ppcle etc...) contain x86_64 binaries. Also notice they have identical size:

To Reproduce
Steps to reproduce the behavior:

1. Setup QEMU on you machine
2. Inspect an image for e.g. arm64:

docker run -it --rm --entrypoint=/bin/bash --platform=linux/arm64 weastur/poetry:1.6.1-python-3.11-slim
Unable to find image 'weastur/poetry:1.6.1-python-3.11-slim' locally
1.6.1-python-3.11-slim: Pulling from weastur/poetry
a378f10b3218: Already exists 
c11bdfacfd25: Already exists 
64fc9a66a5d8: Already exists 
5146634606ba: Already exists 
ea740afc3f32: Already exists 
318dece267be: Already exists 
Digest: sha256:ed52ffbca6cdbea599a67bac374470c8a1f15627fbdff9e99234ac72d480766c
Status: Downloaded newer image for weastur/poetry:1.6.1-python-3.11-slim
root@27c2c22045fc:/# uname -m
x86_64

1. Compare with the results on the official image:

docker run -it --rm --entrypoint=/bin/bash --platform=linux/arm64 python:3.11-slim
Unable to find image 'python:3.11-slim' locally
3.11-slim: Pulling from library/python
24c63b8dcb66: Pull complete 
e7a4e3413405: Pull complete 
faabdb9318a7: Pull complete 
eea1a21f176f: Pull complete 
13ab44602356: Pull complete 
Digest: sha256:fc39d2e68b554c3f0a5cb8a776280c0b3d73b4c04b83dbade835e2a171ca27ef
Status: Downloaded newer image for python:3.11-slim
root@3f5d61055a8a:/# uname -m
aarch64

Expected behavior
Additional images should contain binaries for the right architecture

Additional context
It appears in this commit you should have been using TARGETPLATFORM as opposed to BUILDPLATFORM (or simply drop the --platform option altogether?)