Missing packages on php:7.1-fpm-stretch

[devonhk]

Description
Some debian:stretch apt packages are not available on php:7.1-fpm-stretch even though it is based of the same image.

Repro steps

1. docker run --name php --rm -d php:7.1-fpm-stretch
2. docker exec -it php bash
3. apt update -y
4. apt install php-mysql -y

php:7.1-fpm-stretch

root@87497eb5fa06:/var/www/html# apt install php-mysql -y
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package php-mysql is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'php-mysql' has no installation candidate

root@87497eb5fa06:/var/www/html# apt show php-mysql
Package: php-mysql
State: not a real package (virtual)
N: Can't select candidate version from package php-mysql as it has no candidate
N: There is 1 additional record. Please use the '-a' switch to see it
N: No packages found

debian:stretch

root@e2b7318432d6:/# apt show php-mysql
Package: php-mysql
Version: 1:7.0+49
Priority: optional
Section: php
Source: php-defaults (49)
Maintainer: Debian PHP Maintainers <pkg-php-maint@lists.alioth.debian.org>
Installed-Size: 12.3 kB
Depends: php-common, php7.0-mysql
Download-Size: 5052 B
APT-Manual-Installed: yes
APT-Sources: http://deb.debian.org/debian stretch/main amd64 Packages
Description: MySQL module for PHP [default]

Is this expected behavior?

[wglambert]

That would be due to #542

As noted in the Dockerfile

php/7.1/stretch/fpm/Dockerfile

Lines 7 to 16 in 4017680

	FROM debian:stretch-slim
	# prevent Debian's PHP packages from being installed
	# https://github.com/docker-library/php/pull/542
	RUN set -eux; \
	{ \
	echo 'Package: php*'; \
	echo 'Pin: release *'; \
	echo 'Pin-Priority: -1'; \
	} > /etc/apt/preferences.d/no-debian-php

And the section in the docs about it https://github.com/docker-library/docs/tree/master/php#e-package-php-xxx-has-no-installation-candidate

[devonhk]

@wglambert Thanks for the explanation.