This exposes limited plantuml functionality. You provide as an argument a plantuml file and it will save a .svg and .png file as output.
The easiest way would be to grab the script and add it to /usr/local/bin
or elsewhere in your PATH, e.g.
wget https://raw.githubusercontent.com/0x646e78/docker-plantuml/master/plantuml -O /usr/local/bin/plantuml
You can then run with
plantuml <diagram.puml>
SVG and PNG output will be saved into out/