adding pre-init modification logic

removing ability to build base images on x86 and ripping out single qemu layer

adding pre-init modification logic to pull a tarball of files from dockerhub and apply it to the image if the user passes DOCKER_MODS env variable

Xenial pipeline

bump overlay to v1.21.2.2

build x86 xenial from rootfs like armhf and arm64 for consistency

Add tzdata package

Locale gen

bump s6 to 1.19.1.1

fix donations link

always setting uid and gid rather than test