So I found the Icinga repo for build scripts and dockerfiles. I have Docker running on my Honeycomb, but modifying all that code seemed like more trouble than it was worth.
Hacky solution below. Successfully running 2.13.2 and 2.9.5 on Bullseye on ARM64 now.
FROM debian:bullseye
RUN apt-get update && apt-get dist-upgrade -y
RUN apt-get install build-essential ca-certificates gpg curl -y
RUN echo "deb-src [signed-by=/usr/share/keyrings/icinga-archive-keyring.gpg] http://packages.icinga.com/debian icinga-bullseye main" > /etc/apt/sources.list.d/icinga.list
RUN curl https://packages.icinga.com/icinga.key | gpg --dearmor -o /usr/share/keyrings/icinga-archive-keyring.gpg
RUN apt-get update && mkdir -p /opt/build /opt/debs
ADD build_icinga.sh /opt/build
WORKDIR /opt/build
CMD [ "/bin/bash", "build_icinga.sh" ]
#!/bin/bash
apt-get build-dep icinga2 icingaweb2 icinga-php-common icinga-php-library icinga-php-thirdparty -y
apt-get source icinga2 icingaweb2 icinga-php-common icinga-php-library icinga-php-thirdparty
ICINGADIR=$(find /opt/build -maxdepth 1 -type d | grep icinga2-2)
ICINGAWEBDIR=$(find /opt/build -maxdepth 1 -type d | grep icingaweb2-2)
ICINGAPHPCOMMONDIR=$(find /opt/build -maxdepth 1 -type d | grep icinga-php-common)
ICINGAPHPLIBRARY=$(find /opt/build -maxdepth 1 -type d | grep icinga-php-library)
ICINGAPHPTHIRDPARTY=$(find /opt/build -maxdepth 1 -type d | grep icinga-php-thirdparty)
cd $ICINGADIR
dpkg-buildpackage -b -uc -us
cd ${ICINGAWEBDIR}
dpkg-buildpackage -b -uc -us
cd ${ICINGAPHPCOMMONDIR}
dpkg-buildpackage -b -uc -us
cd ${ICINGAPHPLIBRARY}
dpkg-buildpackage -b -uc -us
cd ${ICINGAPHPTHIRDPARTY}
dpkg-buildpackage -b -uc -us
cd /opt/build
mv *.deb /opt/debs