Docker is used to build official releases, and nightly images in Ubuntu LTS 22.04 Docker containers at the teams https://github.com/LibreELEC/actions CI/CD.
The following links will help you get started with using docker to build LibreELEC.
Build Container
# apply patches to make the image like in CI
sed -i -e "/^USER docker/i RUN ln -s /usr/bin/gcc-10 /usr/bin/cc" tools/docker/focal/Dockerfile
# per tools/docker/README.md
docker build --pull -t libreelec tools/docker/focal