Dockerfile VOLUME not visibile on host - docker

Question

In the WordPress Dockerfile, there's a VOLUME /var/www/html statement. If I understand correctly, this means that the WordPress files (in /var/www/html) should be mapped to the directory on my host containing the docker-compose.yml BUT this is not happening. Do you know why?
I created my own WordPress Dockerfile that extends the original WordPress Dockerfile where you'll find said VOLUME /var/www/html statement on line 44 (https://github.com/docker-library/wordpress/blob/b3739870faafe1886544ddda7d2f2a88882eeb31/php7.2/apache/Dockerfile).
I even tried to add the VOLUME /var/www/html statement at the bottom of my Dockerfile as you can see in my Dockerfile below. I added it just in case but I don't think anything is going wrong in there.
FROM wordpress:4.9.8-php7.2-apache
##########
# XDebug #
##########
# Install
RUN pecl install xdebug-2.6.1; \
docker-php-ext-enable xdebug
# Configure
RUN echo "error_reporting = E_ALL" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini; \
echo "display_startup_errors = On" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini; \
echo "display_errors = On" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini; \
echo "xdebug.idekey=\"PHPSTORM\"" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini; \
echo "xdebug.remote_port=9000" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini; \
echo "xdebug.remote_enable=1" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini; \
echo "xdebug.remote_host=docker.for.win.localhost" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini
#RUN echo "xdebug.remote_autostart=1" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini ##
###########
# PHPUnit #
###########
RUN apt-get update; \
apt-get install wget
RUN wget https://phar.phpunit.de/phpunit-7.4.phar; \
chmod +x phpunit-7.4.phar; \
mv phpunit-7.4.phar /usr/local/bin/phpunit
RUN phpunit --version
###################
# PHP Codesniffer #
###################
RUN curl -OL https://squizlabs.github.io/PHP_CodeSniffer/phpcs.phar; \
mv phpcs.phar /usr/local/bin/phpcs; \
chmod +x /usr/local/bin/phpcs
############
# Composer #
############
RUN php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"; \
php -r "if (hash_file('sha384', 'composer-setup.php') === '93b54496392c062774670ac18b134c3b3a95e5a5e5c8f1a9f115f203b75bf9a129d5daa8ba6a13e2cc8a1da0806388a8') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"; \
php composer-setup.php; \
php -r "unlink('composer-setup.php');"; \
mv composer.phar /usr/local/bin/composer
##################
# Install Nodejs #
##################
RUN apt-get install -y gnupg2; \
curl -sL https://deb.nodesource.com/setup_11.x | bash -; \
apt-get install -y nodejs
##################
# Install Grunt #
##################
RUN npm install -g grunt-cli
#####################
# BASH customization#
#####################
RUN echo "alias ll='ls --color=auto -lA'" >> ~/.bashrc
VOLUME /var/www/html
docker-compose.yml
version: '3'
services:
db:
image: mysql:5.7
volumes:
- ./docker-mysql/db_data:/var/lib/mysql
restart: always
environment:
MYSQL_ROOT_PASSWORD: wordpress
MYSQL_DATABASE: wordpress
MYSQL_USER: wordpress
MYSQL_PASSWORD: wordpress
wordpress:
depends_on:
- db
image: progonkpa/wordpress:1.0
restart: always
ports:
- "80:80"
environment:
WORDPRESS_DB_HOST: db:3306
WORDPRESS_DB_USER: wordpress
WORDPRESS_DB_PASSWORD: wordpress
restart: always
volumes:
db_data:

Answer 1

The volume is being created, it just isn't being created in the execution context where you docker-compose.yml file lives. I assume you are running the ls -lah command and expecting something to be created in the directory where your docker-compose.yml file is. That is why you say, "BUT this is not happening"
The VOLUME command in the Dockerfile is limited. The host is unknown when you build an image from the Dockerfile. It is not until the docker run is executed using your built image that the Docker host is known.
And so, when using the VOLUME command in a Dockerfile and then using docker run with that image, the volume is created in a location configured by the Docker installation. To confirm that a volume has indeed been created for your container use this command:
docker inspect -f '{{ .Mounts }}' [container_name]
To have better control and specify where you VOLUME is created on your Docker host, you need to use the -v option with docker run or configure in your docker-compose.yml file, like is being done for your MySQL persistence container.
You can remove VOLUME /var/www/html from your Dockerfile, and you should. Because your FROM wordpress image creates the VOLUME, as you already know.