The Docker test framework cannot build the office image on ARM64

Functional tests using @UITest(office = true) (e.g. the ones of xwiki-platform-office-test-docker) cannot start on an ARM64 machine (e.g. a Mac with an Apple Silicon CPU), while the other Docker-based tests work there.

The Docker test framework builds a servlet container image embedding LibreOffice, and ServletContainerExecutor always downloads the x86-64 LibreOffice deb packages (deb/x86_64/LibreOffice_<version>_Linux_x86-64_deb.tar.gz). On an ARM64 Docker host the base servlet image is ARM64, so installing them fails and the tests error out in beforeAll:

Could not build image: The command '/bin/sh -c cd `ls -d /tmp/LibreOffice_${LIBREOFFICE_VERSION}*_Linux_x86-64_deb/DEBS` && apt-get install --no-install-recommends ./*.deb && ln -fs `ls -d /opt/libreoffice*` /opt/libreoffice' returned a non-zero code: 100

with, in the image build output:

dpkg: error processing archive /tmp/apt-dpkg-install-3Rh8sJ/00-libreoffice26.2-ure_26.2.6.3-3_amd64.deb (--unpack):
 package architecture (amd64) does not match system (arm64)

LibreOffice also publishes aarch64 deb packages (deb/aarch64/LibreOffice_<version>_Linux_aarch64_deb.tar.gz): the image should use the packages matching the architecture of the Docker host. An image built with them was verified to work (the office functional tests pass on an Apple Silicon Mac).