<error>WARNING: Unattended installation should have a compilation policy.</error>

If any packages require unrecognized compilation steps, composer will
prompt for approval. However, it is not possible to prompt during unattended
installation.

You should set a preference via environment-variable or composer
configuration, e.g.

 - <comment>export COMPOSER_COMPILE=all</comment>
 - <comment>export COMPOSER_COMPILE=whitelist</comment>
 - <comment>export COMPOSER_COMPILE=none</comment>
 - <comment>composer config extra.compile-mode all</comment>
 - <comment>composer config extra.compile-mode whitelist</comment>
 - <comment>composer config extra.compile-mode none</comment>
