Cache common dependencies

Author: magaupp

Dockerfile

@@ -6,3 +6,7 @@ RUN dart pub global activate --source git https://github.com/ls1intum/dart_analy
 RUN apt-get update && apt-get install -y --no-install-recommends \
   xmlstarlet \
   && rm -rf /var/lib/apt/lists/*
+
+# Cache common dependencies
+COPY pubspec.yaml pubspec.lock .
+RUN dart pub get && rm -r .dart_tool pubspec.yaml pubspec.lock