Adds dev container setup with PlantUML support

Introduces a development container using an Ubuntu-based image.
Installs Java, Graphviz, and PlantUML for local diagram rendering.
Sets up a script to start the PlantUML server automatically in interactive sessions.

.devcontainer/Dockerfile

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+FROM mcr.microsoft.com/devcontainers/base:ubuntu
+
+# Install Java and graphviz (for local rendering)
+RUN apt-get update && \
+    apt-get install -y openjdk-17-jre graphviz wget && \
+    wget https://downloads.sourceforge.net/project/plantuml/plantuml.jar -O /usr/local/bin/plantuml.jar
+
+# PlantUML-Startskript in den Home-Ordner legen
+RUN echo '#!/bin/bash\n' \
+         'if ! pgrep -f "plantuml.jar -picoweb" > /dev/null; then\n' \
+         '  nohup java -Djava.awt.headless=true -jar /usr/local/bin/plantuml.jar -picoweb -port 8080 > ~/.plantuml.log 2>&1 &\n' \
+         'fi\n' \
+    > /home/vscode/.start-plantuml.sh && \
+    chmod +x /home/vscode/.start-plantuml.sh
+
+RUN echo '[[ $- == *i* ]] && /home/vscode/.start-plantuml.sh' >> /home/vscode/.bashrc