Thanks @ipetrov for pointing the Debian Science Team OpenCV repo. @standmit just FYI, this is how I managed to build all the .deb installers in an Ubuntu Focal Dockerfile (x86_64).
FROM ubuntu:focal
RUN apt update \
&& export DEBIAN_FRONTEND=noninteractive \
&& apt install -y --no-install-recommends \
build-essential \
ca-certificates \
cmake \
file \
git \
gnupg2 \
unzip \
wget
RUN apt update \
&& export DEBIAN_FRONTEND=noninteractive \
&& apt install -y --no-install-recommends \
ant \
debhelper=13.6ubuntu1~bpo20.04.1 \
default-jdk \
dh-python \
doxygen \
javahelper \
libavcodec-dev \
libavformat-dev \
libdc1394-dev \
libdebhelper-perl=13.6ubuntu1~bpo20.04.1 \
libeigen3-dev \
libgdal-dev \
libgdcm-dev \
libgdcm-tools \
libglu1-mesa-dev \
libgoogle-glog-dev \
libgphoto2-dev \
libgstreamer-plugins-base1.0-dev \
libgstreamer1.0-dev \
libgtk-3-dev \
libjpeg-turbo8-dev \
liblapack-dev \
libleptonica-dev \
libopenexr-dev \
libopenjp2-7-dev \
libpng-dev \
libprotobuf-dev \
libraw1394-dev \
libswresample-dev \
libswscale-dev \
libtbb-dev \
libtesseract-dev \
libtiff-dev \
libv4l-dev \
libva-dev \
libvtk7-dev \
libvtkgdcm-dev \
libwebp-dev \
maven-repo-helper \
ninja-build \
ocl-icd-opencl-dev \
protobuf-compiler \
python3-bs4 \
python3-dev \
python3-numpy
WORKDIR /root
ENV OPENCV_VERSION=4.5.4+dfsg-9
RUN git clone https://salsa.debian.org/science-team/opencv -b debian/${OPENCV_VERSION} \
&& cd opencv \
&& sed -i s/-DINSTALL_C_EXAMPLES=ON/-DINSTALL_C_EXAMPLES=OFF/g debian/rules \
&& sed -i s/libvtk9-dev/libvtk7-dev/g debian/control \
&& dpkg-buildpackage -b -rfakeroot -us -uc