contrib: android: build.sh: fix container user

Resolves:

    "Building outside a git clone is not supported."

contrib/android/build.sh

@@ -86,6 +86,7 @@ docker run --rm \
     -v "$PROJECT_ROOT_OR_FRESHCLONE_ROOT":/home/user/wspace/electrum \
     -v "$PROJECT_ROOT_OR_FRESHCLONE_ROOT"/.buildozer/.gradle:/home/user/.gradle \
     $DOCKER_RUN_FLAGS \
+    --user user \
     --workdir /home/user/wspace/electrum \
     electrum-android-builder-img \
     ./contrib/android/make_apk.sh "$@"