diff --git a/android/README.md b/android/README.md
index 2288196..66aaa29 100644
--- a/android/README.md
+++ b/android/README.md
@@ -65,6 +65,7 @@ carries several Mbit/s per connection (#24).
 
 ```sh
 android/build.sh                    # debug + release APK in Docker (ci/Dockerfile.android)
+VTESSE_ANDROID_IMAGE=<image> android/build.sh   # use a ready image (JDK 17, SDK/NDK/CMake, gradle, ANDROID_HOME)
 adb install -r android/app/build/outputs/apk/debug/app-debug.apk
 ```
 
diff --git a/android/build.sh b/android/build.sh
index c5a14a3..2da89f7 100755
--- a/android/build.sh
+++ b/android/build.sh
@@ -6,6 +6,12 @@
 # Output: android/app/build/outputs/apk/{debug,release}/. Gradle and its
 # dependency cache live in android/.gradle-cache (not in the repository).
 #
+# VTESSE_ANDROID_IMAGE=<image> uses a ready Android image instead of building
+# ci/Dockerfile.android. The image must provide JDK 17, the SDK/NDK/CMake of
+# that Dockerfile, `gradle` on PATH, and export ANDROID_HOME (or
+# ANDROID_SDK_ROOT) - the script checks the last two before building. Without
+# the variable the image is built as vtesse-android.
+#
 # Release signing (optional; without it the release APK is unsigned):
 #   VTESSE_KEYSTORE=/path/to/key.jks plus VTESSE_KEYSTORE_PASSWORD, VTESSE_KEY_ALIAS,
 #   VTESSE_KEY_PASSWORD in the environment (CI: from repository secrets), or
@@ -29,11 +35,20 @@ if [ -n "${VTESSE_KEYSTORE:-}" ]; then
     export VTESSE_KEYSTORE_PASSWORD VTESSE_KEY_ALIAS VTESSE_KEY_PASSWORD
 fi
 
-docker build -q -t vtesse-android -f "$ROOT/ci/Dockerfile.android" "$ROOT/ci" >/dev/null
+IMAGE=${VTESSE_ANDROID_IMAGE:-}
+ENTRY=
+if [ -z "$IMAGE" ]; then
+    IMAGE=vtesse-android
+    docker build -q -t "$IMAGE" -f "$ROOT/ci/Dockerfile.android" "$ROOT/ci" >/dev/null
+else
+    ENTRY=--entrypoint=   # a ready image's own ENTRYPOINT must not swallow the commands
+    docker run --rm $ENTRY "$IMAGE" sh -c 'command -v gradle >/dev/null && [ -n "${ANDROID_HOME:-}${ANDROID_SDK_ROOT:-}" ]' || {
+        echo "$IMAGE: needs gradle on PATH and ANDROID_HOME or ANDROID_SDK_ROOT set" >&2; exit 1; }
+fi
 mkdir -p "$HERE/.gradle-cache/home"
-# shellcheck disable=SC2086  # SIGN is a list of docker options
+# shellcheck disable=SC2086  # SIGN and ENTRY are lists of docker options
 docker run --rm --user "$(id -u):$(id -g)" -e GRADLE_USER_HOME=/src/android/.gradle-cache \
-    -e HOME=/src/android/.gradle-cache/home $SIGN \
-    -v "$ROOT:/src" -w /src/android vtesse-android \
+    -e HOME=/src/android/.gradle-cache/home $SIGN $ENTRY \
+    -v "$ROOT:/src" -w /src/android "$IMAGE" \
     gradle --no-daemon -q ${*:-assembleDebug assembleRelease}
 ls -l "$HERE"/app/build/outputs/apk/*/*.apk