Fix the docker buildx multi-platform builds

Fix the docker buildx multi-platform builds in the .build_docker job.

We have experienced some issues with multi-platform builds:

 > [linux/arm64 fgc-test-manager-builder 5/5] RUN /usr/local/custom-tools/build.sh:
#0 0.336 Error while loading /usr/local/custom-tools/build.sh: Exec format error

Apparently, QEMU and/or binfmt was missing on the runner.

According to the documentation, running a container based on the tonistiigi/binfmt image should configure QEMU and binfmt for multiplatform builds with docker buildx.

A container based on tonistiigi/binfmt will be run in before_script of the .build_docker job and only if the PLATFORMS variable is non-empty.

Closes https://gitlab.cern.ch/kubernetes/project/-/issues/592