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.
