Changes

debian-bookworm-build-arm: FROM: add arm32v7

Use the arm32v7 architecture explicitly, instead of implicitly using the
same architecture as the host. That way we can use the arm32v7 image on
a raspberry pi 4, even if the host system is 64 bit arm.

Using 32 bit arm instead of aarch64 is needed for the osmo-trx jenkins
jobs, which test -mfpu=neon and -mfpu=neon-vfpv4 build flags.

Related: https://github.com/docker-library/official-images#architectures-other-than-amd64
Change-Id: I93986569a9f492237da28fbc534ac5ccd176f120