Description:
------------
This is presumably intentional and temporary, but PECL is not included in the published Docker image: `php:8.0.0alpha1-fpm-alpine` (it also appears to be missing in fpm-buster). For software that depends on pecl packages, this makes testing against the alpha builds difficult/impossible.

Test script:
---------------
$ docker run --rm -it php:8.0.0alpha1-fpm-alpine sh

/var/www/html # apk add $PHPIZE_DEPS && pecl install apcu && docker-php-ext-enable apcu


Alternatively, in a Dockerfile:

FROM php:8.0.0alpha1-fpm-alpine
RUN apk add --update \
  $PHPIZE_DEPS \
  && pecl install apcu \
  && docker-php-ext-enable apcu \
  && && rm -rf /var/cache/apk/*

Expected result:
----------------
Installation completes, or at least returns an error that the pecl packages haven't been updated for PHP 8 yet.

Actual result:
--------------
sh: pecl: not found