问题：

• Channel "pear.php.net" is not responding over http://, failed with message: Connection to ssl://pear.php.net:443' failed:Unable to find the socket transport "ssl" - did you forget to enable it when you configured PHP?
• No releases available for package "pecl.php.net/imagick"

解决方法：

• 删除证书

FROM php:5.6-fpm
# 删除证书
RUN rm /usr/share/ca-certificates/mozilla/DST_Root_CA_X3.crt

• 通过安装包手动安装，需要手动处理php.ini问题

…
&& pecl channel-update pecl.php.net \
&& { \pecl install imagick || ( \wget --no-check-certificate https://pecl.php.net/get/imagick -O ./imagick_latest.tgz \&& pecl install --offline ./imagick_latest.tgz \&& rm ./imagick_latest.tgz \); \
} \
...

附上GitHub issue:
https://github.com/php/php-src/issues/11486