Php – crond: can’t set groups: Operation not permitted

This morning I upgraded my PHP version to 7.1 and am seeing an issue when cron tries to run php /var/www/html/artisan schedule:run (a simple PHP command) I see the output:

3/3/2017 10:39:00 AMcrond: can't set groups: Operation not permitted
3/3/2017 10:39:00 AMcrond: USER www-data pid 1562 cmd php /var/www/html/artisan schedule:run
3/3/2017 10:40:00 AMcrond: can't set groups: Operation not permitted
3/3/2017 10:40:00 AMcrond: USER www-data pid 1563 cmd php /var/www/html/artisan schedule:run
3/3/2017 10:41:00 AMcrond: can't set groups: Operation not permitted
3/3/2017 10:41:00 AMcrond: USER www-data pid 1564 cmd php /var/www/html/artisan schedule:run
3/3/2017 10:42:00 AMcrond: can't set groups: Operation not permitted
3/3/2017 10:42:00 AMcrond: USER www-data pid 1565 cmd php /var/www/html/artisan schedule:run
3/3/2017 10:43:00 AMcrond: can't set groups: Operation not permitted
3/3/2017 10:43:00 AMcrond: USER www-data pid 1566 cmd php /var/www/html/artisan schedule:run

The command being run is a Laravel artisan command. It's run every minute allowing other scheduled work to be completed within the application itself. There's nothing in this command that writes to any files or anything like that. The scheduled work talks to a database and sends some email. Application logs are sent to stdout since it's a Docker container.

cron is run in a container with the command crond -f -d 8. Here's the Dockerfile:

# This container should be used for any/all CLI processes
# including cron, queues, etc.
FROM php:7.1-alpine

# Copy the application files to the container
ADD . /var/www/html

WORKDIR /var/www/html

# fix permissions in CI
RUN sed -ri 's/^www-data:x:82:82:/www-data:x:1000:1000:/' /etc/passwd \
    && sed -ri 's/^www-data:x:82:/www-data:x:1000:/' /etc/group

# Install Composer dependencies
RUN apk add --update --no-cache git zip unzip \

        # needed for spatie/laravel-backup
        mysql-client \

        # needed for gd
        libpng-dev libjpeg-turbo-dev \

    && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

RUN docker-php-ext-install pdo_mysql gd \

        # needed for forking processes in laravel queues as of Laravel 5.3
        pcntl

# Ownership of the app dir for www-data
RUN chown -R www-data:www-data /var/www/html /home/www-data/

# Put php artisan schedule:run in a crontab
RUN echo "*       *       *       *       *       php /var/www/html/artisan schedule:run" > /etc/crontabs/www-data

# Make sure when users get into the container they aren't root
USER www-data

I've ruled out that php artisan schedule:run is the cause since I can run it manually and everything's fine. This means it's something within cron.

What is cron doing under the covers that could cause this error?