Using UBUNTU_VERSION with --build-arg you can choose the Ubuntu version used as base image. The lowest tested, and also the default, is 16.04.
By default HOST_ARCH is set to x86_64
$ docker build -f docker/hashcatToolchain.ubuntu -t gm4tr1x/hashcat-toolchain:x86_64-16.04 .
Must specify HOST_ARCH as aarch64
$ docker build --build-arg HOST_ARCH=aarch64 -f docker/hashcatToolchain.ubuntu -t gm4tr1x/hashcat-toolchain:aarch64-16.04 .