feat(devcontainer): improve plantuml install and cleanup steps

.devcontainer/Dockerfile

@@ -1,9 +1,16 @@
 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
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+    openjdk-17-jre \
+    graphviz \
+    curl \
+    && curl -L \
+    https://github.com/plantuml/plantuml/releases/latest/download/plantuml.jar \
+    -o /usr/local/bin/plantuml.jar \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*
 
 # PlantUML-Startskript in den Home-Ordner legen
 RUN echo '#!/bin/bash\n' \