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Cron cannot modify header information (wordpress docker)

I’m following this tutorial: WordPress developer’s intro to Docker, part two
When I run docker-compose up after it start everything, I see this error message:

cron_1 | Warning: Cannot modify header information - headers already sent by (output started at /usr/src/wordpress/wp-includes/load.php:18) in /usr/src/wordpress/wp-includes/pluggable.php on line 1171

Since my cloud is Ubuntu Docker 1.12.4 on 16.04, I’ve tried to change the following directives
COPY cron.conf /etc/crontabs/deployer
RUN chmod 600 /etc/crontabs/deployer
COPY cron.conf /var/spool/cron/crontabs/deployer
RUN chmod 600 /var/spool/cron/crontabs/deployer

but the problem still exist. How to solve this problem?