plantuml is open source, and that includes its Dockerfile. https://github.com/plantuml/plantuml/blob/1427bb5acccdce67bc4730671b253bbe9b1a80e1/Dockerfile
If you cannot docker pull from the registry due to corporate policy, then you can "copy paste" the Dockerfile and make adaptations. In particular, in place of this wget:
RUN wget \
"https://github.com/plantuml/plantuml/releases/download/${PLANTUML_VERSION}/plantuml-${PLANTUML_VERSION#?}.jar" \
-O /opt/plantuml.jar
you can download the plantuml.jar file "outside" then COPY it into the docker image. Moreover, in this case, you can further "optimize" it because you don't need the multi-stage build anymore (but you'd have likely figured that out).
You need to take a couple of things into account though:
1. How often do you want to update the plantuml image? You might need an "outer" script to automate the download the plantuml.jar then build the docker image.
2. Do you absolutely need to use Docker? Can you not just make do with "java -jar plantuml.jar ..."?
3. Specifically because you mentioned corporate policy, you should consider the license. Silly as it may sound, in my experience, "copy pasting" is serious business for legal teams in software companies. Additionally, among the various Open Source licenses, the GNU GPL v3 (https://github.com/plantuml/plantuml/blob/master/COPYING) is often touched with a ten-metre pole in proprietary software development. (Disclaimer: I am not a lawyer.)
