add imagemagick-svg to fpm-alpine Dockerfile

[felix-egli]

this adds support for SVG images.

[jessebot]

@joshtrichards does it need to go here in this apk add section: https://github.com/nextcloud/docker/blob/f45486780338805720417c023ed2ccba88f8536e/Dockerfile-alpine.template#L18-L36

or here in this second one: https://github.com/nextcloud/docker/blob/f45486780338805720417c023ed2ccba88f8536e/Dockerfile-alpine.template#L118-L121

[joshtrichards]

I'd probably just put it here (same as @felix-egli did in the 29 Dockerfile):

https://github.com/nextcloud/docker/blob/f45486780338805720417c023ed2ccba88f8536e/Dockerfile-alpine.template#L6-L8

[joshtrichards]

Superceded by #2246. Thanks for getting the process started, @felix-egli!