Add imagemagick

[minch4]

Hello o/

It would be nice to have imagemagick natively integrated in this image.
Asciidoctor Diagram need it for some tools like the Meme Extension.

Failed to generate image: Could not find the 'convert' executable in PATH; add it to the PATH or specify its location using the 'convert' document attribute

[barthel]

I'm building a full blown Asciidoctor Container Image with all Asciidoctor Diagrams extensions.
Imagemagick (for meme) is already implemented.
Give it a try.

Github: https://github.com/barthel/docker-asciidoctor
Docker Hub: https://hub.docker.com/r/uwebarthel/asciidoctor