This is the PHP5 extension for ImageMagick as an Alpine Linux package.
The package provided by Alpine Linux was removed from their repositories in the 3.6 release. A package for PHP7 is still available there.
See the releases page for the latest download links.
The current installation method for these packages is to pull them in using
wget or curl and install the local file with apk:
apk --no-cache add ca-certificates wget
wget -q -O /etc/apk/keys/sgerrand.rsa.pub https://alpine-pkgs.sgerrand.com/sgerrand.rsa.pub
wget https://github.com/sgerrand/alpine-pkg-php5-imagick/releases/download/3.4.4-r0/php5-imagick-3.4.4-r0.apk
apk add php5-imagick-3.4.4-r0.apk