diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 3e0cbd4..82f7a93 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -11,8 +11,19 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    container:
+      image: docker.gitea.com/runner-images:ubuntu-latest
+      # A directory on the runner host, whitelisted in the runner's
+      # valid_volumes. Everything the job would otherwise download on every
+      # push — JDK, Android SDK, Gradle distribution and dependencies —
+      # lives here and survives the container.
+      options: -v /etc/komodo/stacks/gitea/runner-cache/castarr:/cache
     env:
-      ANDROID_SDK_ROOT: /opt/android-sdk
+      CACHE: /cache
+      JAVA_HOME: /cache/jdk17
+      ANDROID_SDK_ROOT: /cache/android-sdk
+      GRADLE_USER_HOME: /cache/gradle
+      JDK_URL: https://api.adoptium.net/v3/binary/latest/17/ga/linux/x64/jdk/hotspot/normal/eclipse
       CMDLINE_TOOLS: https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip
     steps:
       - name: Checkout
@@ -22,35 +33,42 @@ jobs:
           git fetch -q --depth 1 origin "${GITHUB_SHA}"
           git checkout -q FETCH_HEAD
 
-      - name: Install JDK
+      - name: JDK 17
         run: |
-          apt-get update -qq
-          DEBIAN_FRONTEND=noninteractive apt-get install -y -qq openjdk-17-jdk-headless > /dev/null
-          java -version
+          if [ ! -x "$JAVA_HOME/bin/java" ]; then
+            echo "JDK nicht im Cache — einmalig herunterladen"
+            mkdir -p "$JAVA_HOME"
+            curl -sSL "$JDK_URL" | tar -xz -C "$JAVA_HOME" --strip-components=1
+          fi
+          "$JAVA_HOME/bin/java" -version
 
-      - name: Install Android SDK
+      - name: Android SDK
         run: |
-          mkdir -p "$ANDROID_SDK_ROOT/cmdline-tools"
-          curl -sSL -o /tmp/tools.zip "$CMDLINE_TOOLS"
-          unzip -q /tmp/tools.zip -d "$ANDROID_SDK_ROOT/cmdline-tools"
-          mv "$ANDROID_SDK_ROOT/cmdline-tools/cmdline-tools" "$ANDROID_SDK_ROOT/cmdline-tools/latest"
-          # Accept licences by writing the hashes: piping "yes" into
-          # sdkmanager dies of SIGPIPE (exit 141) under pipefail.
-          mkdir -p "$ANDROID_SDK_ROOT/licenses"
-          echo "24333f8a63b6825ea9c5514f83c2829b004d1fee" > "$ANDROID_SDK_ROOT/licenses/android-sdk-license"
-          echo "84831b9409646a918e30573bab4c9c91346d8abd" > "$ANDROID_SDK_ROOT/licenses/android-sdk-preview-license"
-          "$ANDROID_SDK_ROOT/cmdline-tools/latest/bin/sdkmanager" \
-            "platforms;android-35" "build-tools;35.0.0" "platform-tools" > /dev/null
+          if [ ! -d "$ANDROID_SDK_ROOT/platforms/android-35" ]; then
+            echo "SDK nicht im Cache — einmalig einrichten"
+            mkdir -p "$ANDROID_SDK_ROOT/cmdline-tools"
+            curl -sSL -o /tmp/tools.zip "$CMDLINE_TOOLS"
+            unzip -q -o /tmp/tools.zip -d "$ANDROID_SDK_ROOT/cmdline-tools"
+            rm -rf "$ANDROID_SDK_ROOT/cmdline-tools/latest"
+            mv "$ANDROID_SDK_ROOT/cmdline-tools/cmdline-tools" "$ANDROID_SDK_ROOT/cmdline-tools/latest"
+            # Accept licences by writing the hashes: piping "yes" into
+            # sdkmanager dies of SIGPIPE (exit 141) under pipefail.
+            mkdir -p "$ANDROID_SDK_ROOT/licenses"
+            echo "24333f8a63b6825ea9c5514f83c2829b004d1fee" > "$ANDROID_SDK_ROOT/licenses/android-sdk-license"
+            echo "84831b9409646a918e30573bab4c9c91346d8abd" > "$ANDROID_SDK_ROOT/licenses/android-sdk-preview-license"
+            JAVA_HOME="$JAVA_HOME" "$ANDROID_SDK_ROOT/cmdline-tools/latest/bin/sdkmanager" \
+              "platforms;android-35" "build-tools;35.0.0" "platform-tools" > /dev/null
+          fi
           echo "sdk.dir=$ANDROID_SDK_ROOT" > local.properties
 
-      - name: Unit tests
-        run: ./gradlew testDebugUnitTest --no-daemon --stacktrace
-
-      - name: Build debug APK
-        run: ./gradlew assembleDebug --no-daemon --stacktrace
+      # One invocation, not two: a second --no-daemon run pays for another JVM
+      # start and another configuration phase to redo work it just did.
+      - name: Tests und Debug-APK
+        run: ./gradlew testDebugUnitTest assembleDebug --no-daemon --stacktrace
 
       - name: Summary
         if: always()
         run: |
           echo "APK:"; ls -la app/build/outputs/apk/debug/ 2>/dev/null || echo "  (kein Build)"
           echo "Tests:"; ls tests/runs/junit/ 2>/dev/null || echo "  (keine Reports)"
+          echo "Cache:"; du -sh "$CACHE"/* 2>/dev/null || echo "  (leer)"