Docker www-data grant write permissions to var/www - apache

Question

I'm running a php:7.0-apache Docker image, but I have no permissions to write to /var/www/html. How is it possible to grant write rights to this user?
Dockerfile:
FROM php:7.0-apache
# PHP Extensions
RUN docker-php-ext-install pdo_mysql
# Composer
RUN php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
RUN php composer-setup.php
RUN php -r "unlink('composer-setup.php');"
RUN mv composer.phar /usr/local/bin/composer
ADD vhost-default.conf /etc/apache2/sites-enabled/000-default.conf
# Open Ports
EXPOSE 80
EXPOSE 443
Host Conf
<VirtualHost *:80>
ServerAdmin webmaster#localhost
DocumentRoot /var/www/html
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
<Directory /var/www/html >
Options Indexes FollowSymLinks
AllowOverride All
Require all granted
</Directory>
</VirtualHost>

Answer 1

To resolve this you will need to add an extra line in your Dockerfile like:
RUN chown www-data:www-data /var/www/html/

Answer 2

Sergiu's answer is not working, because your volume is bound after chown.
What you should do is that you should run chown after bound to your volume and before start the Apache.
To do that, I add chown command in the apache2-foreground script.
RUN sed -i 's/^exec /chown www-data:www-data \/var\/www\/html/\n\nexec /' /usr/local/bin/apache2-foreground
So when you start your container, it will change the permission to www-data.
PS: Your container's user should be in root or a user who can execute chown.