ci(.gitlab-ci): use a Docker image for Decker builds

.gitlab-ci.yml

@@ -28,30 +28,15 @@ prebuild:
     expire_in: "1 hour"
 
 build:
-  image: ubuntu:24.04
+  image: registry.gitlab.fachschaften.org/robinst/decker-image:1.0.2
   stage: build
-  cache:
-    key: decker-cache
-    paths:
-      - .cache/decker/
-    policy: pull-push
   before_script:
-    - mkdir -p .cache/decker
-    - |
-      if [ ! -f .cache/decker/decker ]; then
-        echo "Downloading decker..."
-        apt-get update && apt-get install -y curl
-        curl -L -o .cache/decker/decker https://github.com/decker-edu/decker/releases/download/latest/decker-latest-Linux-X64-3f4150d61ca167361b02665da969da769ca98e7c
-        chmod +x .cache/decker/decker
-      else
-        echo "Using cached decker binary"
-      fi
   dependencies:
     - prebuild
   script: |
     cd $PREBUILD_OUTPUT
     echo "Generate Decks with Decker."
-    ../.cache/decker/decker decks
+    /usr/local/bin/decker decks
     echo "Successfully built Decker slides."
     cp fvv-folien.pdf public/fvv-folien.pdf
   artifacts: