The Docker image fails to build without BuildKit because TARGETARCH is empty

Problem

The Dockerfile installs a controlled LibreOffice version and selects the download matching the image
architecture by branching on the TARGETARCH build argument.

TARGETARCH is only populated automatically by BuildKit. The Docker Official Images build
infrastructure uses the classic builder, where an ARG TARGETARCH declaration with no
--build-arg yields an empty value. The architecture selection therefore falls into its error branch
and the build fails:

Step 14/33 : RUN case "$TARGETARCH" in     amd64) ...
Unsupported target architecture [] for the LibreOffice installation

This broke the Docker Hub update for XWiki 18.6.0: all three 18-* variants failed to build. The
same failure happens to any user building the image locally with BuildKit disabled
(DOCKER_BUILDKIT=0).

All generated Dockerfile}}s are affected. Only the {{18 ones failed so far because the other cycles
are still published from commits predating the LibreOffice installation step; bumping any of them
would fail the same way.

Fix

Read the architecture from the image itself with dpkg --print-architecture instead of relying on a
build argument. This is the approach Docker Official Images recommends, it yields the same
amd64/arm64 values, and it works whether or not BuildKit is in use.