A Dockerfile doesn't install RUN dependencies - php

Question

docker-compose.yml
version: '3.7'
services:
php:
build:
context: .
dockerfile: Dockerfile
image: php:7.3-rc-fpm
container_name: php_7.3-rc-fpm
volumes:
- .:/var/www/app
restart: unless-stopped
working_dir: /var/www
stdin_open: true
tty: true
Dockerfile
FROM php:7.3-rc-fpm
RUN apt-get update && apt-get install -y \
build-essential \
mysql-client \
locales \
zip \
vim \
unzip \
git \
curl
# Clear cache
RUN apt-get clean && rm -rf /var/lib/apt/lists/*
# Install extensions
RUN docker-php-ext-install pdo_mysql mbstring zip pcntl
# Install composer
RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
# Copy existing application directory permissions
COPY --chown=www:www . /var/www
# Change current user to www
USER www
Started containers with
docker-compose up -d
and when I execute
docker-compose exec php bash
followed by
mysql --version
result in
bash: mysql: command not found
the mysql-client is missing and the others RUNs installation as well...
Any idea what is going on?
....and stackoverflow need more details to approve my edit when I don't have any ............

Answer 1

image: php:7.3-rc-fpm should be dropped.
It tells docker-compose to build from the "php_7.3-rc-fpm" image and not from the image build with your Dockerfile (it's a question of precedence). So it's normal that nothing you ask to install in the Dockerfile is available...
I tested to be sure and indeed, if you drop this line, the commands docker-compose exec php bash followed by mysql --version gives you what you expected.

Answer 2

You are misusing container image name. In your docker-compose.yml you tell:
services:
php:
build:
context: .
dockerfile: Dockerfile
image: php:7.3-rc-fpm
That you want to build your own image and name it php:7.3-rc-fpm! But this is not your image' name - it is the name of a well-known php docker container! And in your Dockerfile you inherits from it:
FROM php:7.3-rc-fpm
So, you are overwriting public image but your own. And I can only guess, what will be the new image like..
Solution - remove image from your docker-compose file. It is not the image to be used, it is the name you want to give your image after being built, upon used in conjunction with build properties.