Can't seems to be able to apt install php-* packages on php:7.1.20-fpm-stretch

[b-enoit-be]

Example with php-apcu

$ docker run -ti php:7.1.20-fpm-stretch bash

root@344ee0459d47:/var/www/html# apt update && apt install php-apcu
Get:3 http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]
Ign:1 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Get:2 http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease [91.0 kB]
Get:4 http://cdn-fastly.deb.debian.org/debian stretch Release [118 kB]
Get:5 http://cdn-fastly.deb.debian.org/debian stretch Release.gpg [2434 B]
Get:6 http://cdn-fastly.deb.debian.org/debian stretch-updates/main amd64 Packages [5148 B]
Get:7 http://security.debian.org/debian-security stretch/updates/main amd64 Packages [376 kB]
Get:8 http://cdn-fastly.deb.debian.org/debian stretch/main amd64 Packages [7099 kB]
Fetched 7785 kB in 3s (2186 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package php-apcu 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-apcu' has no installation candidate

root@344ee0459d47:/var/www/html# apt search php-apcu
Sorting... Done
Full Text Search... Done
php-apcu/stable 5.1.8+4.0.11-1 amd64
  (none)

php-apcu-bc/stable 1.0.3-2 amd64
  (none)

root@344ee0459d47:/var/www/html# apt update && apt install php-apcu/stable
Hit:3 http://security.debian.org/debian-security stretch/updates InRelease
Ign:1 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Hit:2 http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease
Hit:4 http://cdn-fastly.deb.debian.org/debian stretch Release
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
Reading package lists... Done
Building dependency tree
Reading state information... Done
Selected version '5.1.8+4.0.11-1' (Debian:9.5/stable [amd64]) for 'php-apcu'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 php-apcu : Depends: phpapi-20151012
            Recommends: php-apcu-bc but it is not installable
E: Unable to correct problems, you have held broken packages.

Other one on php-xdebug

root@344ee0459d47:/var/www/html# apt install php-xdebug/stable
Reading package lists... Done
Building dependency tree
Reading state information... Done
Selected version '2.5.0-1' (Debian:9.5/stable [amd64]) for 'php-xdebug'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 php-xdebug : Depends: phpapi-20151012
E: Unable to correct problems, you have held broken packages.

The same on the base debian:stretch-slim, seems to work, though;

$ docker run -ti debian:stretch-slim bash
root@83dfd3155a33:/# apt update && apt install php-apcu
Get:3 http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]
Ign:1 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Get:2 http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease [91.0 kB]
Get:4 http://cdn-fastly.deb.debian.org/debian stretch Release [118 kB]
Get:5 http://cdn-fastly.deb.debian.org/debian stretch Release.gpg [2434 B]
Get:6 http://cdn-fastly.deb.debian.org/debian stretch-updates/main amd64 Packages [5148 B]
Get:7 http://security.debian.org/debian-security stretch/updates/main amd64 Packages [376 kB]
Get:8 http://cdn-fastly.deb.debian.org/debian stretch/main amd64 Packages [7099 kB]
Fetched 7785 kB in 2s (2624 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
  bzip2 file libbsd0 libedit2 libgpm2 libicu57 libmagic-mgc libmagic1 libncurses5 libssl1.1 libxml2 mime-support php-apcu-bc php-common
  php7.0-cli php7.0-common php7.0-json php7.0-opcache php7.0-phpdbg php7.0-readline psmisc sgml-base ucf xml-core xz-utils
Suggested packages:
  bzip2-doc gpm php-gd php-pear sgml-base-doc debhelper
The following NEW packages will be installed:
  bzip2 file libbsd0 libedit2 libgpm2 libicu57 libmagic-mgc libmagic1 libncurses5 libssl1.1 libxml2 mime-support php-apcu php-apcu-bc
  php-common php7.0-cli php7.0-common php7.0-json php7.0-opcache php7.0-phpdbg php7.0-readline psmisc sgml-base ucf xml-core xz-utils
0 upgraded, 26 newly installed, 0 to remove and 0 not upgraded.
Need to get 14.9 MB of archives.
After this operation, 58.8 MB of additional disk space will be used.
Do you want to continue? [Y/n]

[tianon]

Yep, this is intentional -- you don't want to use Debian's PHP packages with this image (or you don't want to use this image and instead want to use Debian's PHP packages exclusively).

See #542 for the PR which implemented this, which includes many, many links to similar discussions.

[b-enoit-be]

@tianon That is something I totally understand but what I would have expected (maybe in a too simplistic view) was to have the trusted packages added (e.g. from https://packages.sury.org/php/) and then to be able to install it properly.

Or is there something wrong in this idea ?

[tianon]

Surý's packages are great, but they're not officially provided by upstream (or officially recommended from upstream's documentation), which is one of the primary focuses of the Docker official images program.

If you want to use Surý's packages, I'd recommend basing your image on debian:stretch directly and installing from there, since those packages will not be compatible with the PHP provided by this image (and removing the block that #542 installs, while allowing you to install the packages successfully, will not do what you expect, which is the reason for the block -- what will instead happen is that you'll have at least two separate installations of PHP, and will then see subtle behavioral differences depending on which one ends up getting used).

[b-enoit-be]

Makes all the sense then, thanks @tianon