CI: cache the JDK, the SDK and Gradle on the runner host

Every push re-installed a JDK, re-downloaded the Android SDK and let Gradle
fetch its distribution and every dependency again — roughly two of the three
minutes a run took, spent on bytes that never change.

The job now mounts a directory from the runner host at /cache and keeps the
JDK, the SDK and GRADLE_USER_HOME there; both setup steps are skipped when
what they install is already present. Tests and the APK build share one
Gradle invocation instead of paying twice for JVM startup and configuration.

Needs the mount whitelisted in the runner's valid_volumes
(/etc/komodo/stacks/gitea/runner-cache/** on diana).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.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)"