PHP Docker container exits for no reason - php

Question

I'm relatively new to docker and docker-compose so I made this file
version: '3'
services:
web:
image: nginx:latest
ports:
- "8050:80"
volumes:
- ./code:/code
- ./site.conf:/etc/nginx/conf.d/default.conf
links:
- php
php:
image: php:7.3-fpm-alpine3.9
command: apk --update add php7-mysqli
volumes:
- ./code:/code
db:
image: mysql
command: --default-authentication-plugin=mysql_native_password
restart: always
ports:
- 3306:3306
environment:
MYSQL_ROOT_PASSWORD: example
adminer:
image: adminer
restart: always
ports:
- 8080:8080
For some reason the line
command: apk --update add php7-mysqli
Stops php container for no reason, just prints dock_php_1 exited with code 0
Thus my web container also stops and service doesn't work
What could be core of the problem and how to fix it?

Answer 1

This is because you are telling the container to run the apk update command when it starts, this completes and exits with a valid exit code 0...
To get it to run that apk update command and still use the php container, you need to extend the php image with your own build to create your own 'custom image' of the base php image (kinda like extending a PHP class), and then run that apk update as part of the dockerfile.
This is reasonably easy to do and your dockerfile would look something like:
FROM php:7.3-fpm-alpine3.9
RUN apk --update add php7-mysqli
You can save this as ./php/Dockerfile
Then update your docker-compose.yml file to say:
...
php:
build: ./php
volumes:
...
Removing the command: section
This would then, upon docker-compose up, build your extended image with the apk update inside it as an extra layer on the container, and continue running the standard php command that the original image provides.
Here is the documentation on the build: directive, as there are quite a few other cool things you can do with it,
like specifying the Dockerfile if you don't want to put it into a subdirectory, and providing the context: should you wish to bake in files to your new image
https://docs.docker.com/compose/compose-file/#build

Answer 2

I assume what you actually need is to use Docker Compose "build" option, and provide Dockerfile which installing the desired package (RUN apk --update add php7-mysqli). Alternatively, you can build a new Docker image and use it directly in your Docker Compose file.
Docker Compose "command" is to override the default Docker image command (CMD command of the Dockerfile)
More explanation can be found here:
https://medium.freecodecamp.org/docker-entrypoint-cmd-dockerfile-best-practices-abc591c30e21