fix(debian_jdk17) revert #310 and use TARGETPLATFORM to detect if arm/v7 or not

[dduportal]

Related to #323 and #308.

This PR reverts #310 and fixes #308 with a similar approach but with the "conditionnality" of the arm/v7 directly inside the Dockerfile 's jre-build stage.

While reading https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/, I confirmed @carpnick 's analysis about the BUILDPLATFORM. But it seems that Docker provides another variable which is TARGETPLATFORM which is populated with the value we expect.

https://docs.docker.com/engine/reference/builder/#automatic-platform-args-in-the-global-scope shows the documentation reference with an exemple on "how to" use this variable as a build argument: is is what this PR does.

Tested locally with a local registry and a "smoke test":

# Start a local registry to test the `--push` option
$ docker run -d -p 5000:5000 --restart=always --name registry registry:2

# A buildx runner is required to test multi-arch, but it's allowed to reach the `localhost:5000` of the Docker Engine which points to the registry through the Docker port forwarding
$ docker buildx create --use --driver-opt network=host 

# Build and push to the local registry
$ REGISTRY=localhost:5000 JENKINS_REPO=agent docker buildx bake --file docker-bake.hcl --push debian_jdk17

# Smoke tests
for arch in 'linux/amd64' 'linux/arm64' 'linux/arm/v7'; do echo "==$arch=="; docker run --platform="$arch" --rm --entrypoint='' localhost:5000/agent:jdk17 sh -c 'uname -m && java -version';done
==linux/amd64==
Unable to find image 'localhost:5000/agent:jdk17' locally
jdk17: Pulling from agent
Digest: sha256:385f9ebe6a724ac32447eaab2c945fef3d0db1bad558a50b7ae39c765b33a7e9
Status: Downloaded newer image for localhost:5000/agent:jdk17
x86_64
openjdk version "17.0.5" 2022-10-18
OpenJDK Runtime Environment Temurin-17.0.5+8 (build 17.0.5+8)
OpenJDK 64-Bit Server VM Temurin-17.0.5+8 (build 17.0.5+8, mixed mode)
==linux/arm64==
Unable to find image 'localhost:5000/agent:jdk17' locally
jdk17: Pulling from agent
Digest: sha256:385f9ebe6a724ac32447eaab2c945fef3d0db1bad558a50b7ae39c765b33a7e9
Status: Downloaded newer image for localhost:5000/agent:jdk17
aarch64
openjdk version "17.0.5" 2022-10-18
OpenJDK Runtime Environment Temurin-17.0.5+8 (build 17.0.5+8)
OpenJDK 64-Bit Server VM Temurin-17.0.5+8 (build 17.0.5+8, mixed mode)
==linux/arm/v7==
Unable to find image 'localhost:5000/agent:jdk17' locally
jdk17: Pulling from agent
Digest: sha256:385f9ebe6a724ac32447eaab2c945fef3d0db1bad558a50b7ae39c765b33a7e9
Status: Downloaded newer image for localhost:5000/agent:jdk17
armv7l
openjdk version "17.0.5" 2022-10-18
OpenJDK Runtime Environment Temurin-17.0.5+8 (build 17.0.5+8)
OpenJDK Server VM Temurin-17.0.5+8 (build 17.0.5+8, mixed mode, sharing)

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