Add PHP 8.6-rc support

[jaydrogers]

Blocker

redis is the only extension that won't build on PHP 8.6. Everything else works.

phpredis's latest stable is 6.3.0 (2025-11-06), which predates 8.6. The 8.6 fixes are merged to master (phpredis/phpredis#2884, phpredis/phpredis#2888, phpredis/phpredis#2900) but not yet tagged in a release, so PECL still serves 6.3.0.

Same blocker that stopped 8.5-rc (phpredis/phpredis#2688).

Verify

docker run --rm php:8.6-rc-cli-trixie sh -c '
curl -sSLf -o /usr/local/bin/install-php-extensions \
  https://github.com/mlocati/docker-php-extension-installer/releases/download/2.11.12/install-php-extensions
chmod +x /usr/local/bin/install-php-extensions
for e in opcache pcntl pdo_mysql pdo_pgsql zip redis; do
  install-php-extensions "$e" >/dev/null 2>&1 && echo "OK $e" || echo "FAIL $e"
done'

Today: all OK except FAIL redis.

When unblocked

• Bump PHP_EXT_INSTALLER_VERSION in src/common/usr/local/bin/docker-php-serversideup-install-php-ext-installer from 2.9.27 (it rejects 8.6 outright) to 2.11.12+
• Add to scripts/conf/php-versions-base-config.yml under major: "8":

      - minor: "8.6-rc"
        base_os:
          - name: alpine3.23
          - name: alpine3.24
          - name: bookworm
          - name: trixie
        patch_versions:
          - 8.6-rc

• Confirm RC builds don't claim latest or stable minor/major aliases
• Build all five variations on one Debian and one Alpine base

Notes

All 12 upstream base images already exist (cli/fpm/zts × our four bases). Matrix and tag tooling already handle -rc.

patch_versions must be hardcoded — php.net's active.php feed doesn't carry RCs.