Today I learned that I can use Buildah to mount a container image and install packages on top of it from the host.
microcontainer=$(buildah from registry.access.redhat.com/ubi8/ubi-micro)
micromount=$(buildah mount $microcontainer)
yum install \
--installroot $micromount \
--releasever 8 \
--setopt install_weak_deps=false \
--nodocs -y
httpd
yum clean all \
--installroot $micromount
buildah umount $microcontainer
buildah commit $microcontainer ubi-micro-httpd