Use imagick/imagick@master instead of imagick

Building `imagick` is currently unstable.

See Imagick/imagick#640 (comment)

Dockerfile

@@ -16,7 +16,7 @@ FROM mlocati/php-extension-installer:2@sha256:c1b0b8a4c7d2d4077e4c0caf7f6f051256
 FROM php${php}
 
 ARG php=${php}
-ARG php_enable_extensions="apcu bcmath calendar ctype curl dom exif fileinfo ftp gd gettext iconv imagick intl json mbstring memcache memcached mysqli mysqlnd opcache pdo pdo_mysql pdo_sqlite phar posix readline redis shmop simplexml soap sockets sqlite3 sysvmsg sysvsem sysvshm tokenizer xml xmlreader xmlwriter xsl zip"
+ARG php_enable_extensions="apcu bcmath calendar ctype curl dom exif fileinfo ftp gd gettext iconv imagick/imagick@master intl json mbstring memcache memcached mysqli mysqlnd opcache pdo pdo_mysql pdo_sqlite phar posix readline redis shmop simplexml soap sockets sqlite3 sysvmsg sysvsem sysvshm tokenizer xml xmlreader xmlwriter xsl zip"
 ARG php_install_extensions="blackfire xdebug"
 
 HEALTHCHECK --interval=10s --start-period=90s CMD netstat -ltn | grep -c ":9000"

bin/check-doc.sh

@@ -5,7 +5,7 @@ status=0
 trap '{ (( status++ )) ; }' ERR
 
 linenumber=$(grep -n 'ARG php_enable_extensions' Dockerfile | cut -f 1 -d :)
-for extension in $(grep 'ARG php_enable_extensions' Dockerfile | cut -f 2 -d = | tr --delete '"'); do
+for extension in $(grep 'ARG php_enable_extensions' Dockerfile | cut -f 2 -d = | tr --delete '"' | sed -E 's#.*/(.*)@.*#\1#'); do
 	grep -q -- "- ${extension}\$" README.md || (
 		echo "::error file=Dockerfile,line=${linenumber}::PHP extension '${extension}' is not documented in README.md."
 		exit 1