Add helpful comments explaining ruby-build inner workings

unset RUBYOPT

unset RUBYLIB

# If something goes wrong during building, print error information to stderr.

trap build_failed ERR

# This is where the magic happens: execute commands from the definition

# file while in a temporary build directory. This will typically result in

# `install_package` leading to `build_package_standard`.

mkdir -p "$BUILD_PATH"

# shellcheck disable=SC1090

source "$DEFINITION_PATH"

# By default, the temporary build path is wiped after successful build.

[ -z "${KEEP_BUILD_PATH}" ] && rm -fr "$BUILD_PATH"

trap - ERR