build-support/travis/docker_build_image.mustache

docker build
--rm -t ${docker_image_name}
--build-arg "TRAVIS_USER=$(id -un)"
--build-arg "TRAVIS_UID=$(id -u)"
--build-arg "TRAVIS_GROUP=$(id -gn)"
--build-arg "TRAVIS_GID=$(id -g)"
{{! $docker_image_name is an environment variable with an expected value like `travis_ci`.
It should be set in the env section.}}
build-support/docker/${docker_image_name}/