Use fixed APK signing key in CI

Makefile

@@ -9,6 +9,16 @@ APP_VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo
 GO_LDFLAGS ?= -X main.appVersion=$(APP_VERSION)
 ANDROID_MIN_SDK ?= 28
 ANDROID_TARGET_SDK ?= 35
+SIGNKEY ?=
+SIGNPASS ?=
+
+GOGIO_SIGN_FLAGS :=
+ifneq ($(strip $(SIGNKEY)),)
+GOGIO_SIGN_FLAGS += -signkey $(SIGNKEY)
+endif
+ifneq ($(strip $(SIGNPASS)),)
+GOGIO_SIGN_FLAGS += -signpass $(SIGNPASS)
+endif
 
 .PHONY: apk
 apk: android/keepassgo-android.jar
@@ -27,6 +37,7 @@ apk: android/keepassgo-android.jar
 		-buildmode exe \
 		-appid $(APP_ID) \
 		-ldflags "$(GO_LDFLAGS)" \
+		$(GOGIO_SIGN_FLAGS) \
 		-o $(APK_OUT) \
 		-version $(APK_VERSION) \
 		-minsdk $(ANDROID_MIN_SDK) \