Add Debian Buster image with 32-bit libraries

debian:buster:ia32:docker CI image is based on amd64 Debian Buster, but
has additional 32-bit libraries necessary to cross-compile BIND in
32-bit.

As the image name consists of VENDOR-VERSION-ARCH triplet, ARCH needs to
be set to something else than amd64 (we already have
debian-buster-amd64), or i386 as that is misleading, because this image
is not based on i386 Docker image. ia32 ARCH is chosen for this type of
image.

As ia32 is not a standard Docker architecture, it needs to be stripped
from SOURCE_IMAGE variable.

Special variables were added to install 32-bit libraries of DNSTAP
dependencies and the libedit library.
6 jobs for !96 with mnowak/support-cross-compilation-on-buster in 17 minutes and 1 second (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Images
passed #1482155
amd64 linux docker
debian:buster:amd64:docker

00:17:01

passed #1482154
amd64 linux docker
debian:buster:ia32:docker

00:06:35

passed #1482156
amd64 linux docker
debian:sid:amd64:docker

00:04:41

passed #1482153
amd64 linux docker
debian:stretch:amd64:docker

00:06:30

passed #1482158
amd64 linux docker
ubuntu:focal:amd64:docker

00:04:51

passed #1482157
amd64 linux docker
ubuntu:xenial:amd64:docker

00:05:00