add signing config for release builds

app/build.gradle

@@ -17,12 +17,22 @@ android {
         }
     }
 
+    signingConfigs {
+        release {
+            storeFile file('../store.jks')
+            storePassword System.getenv('STORE_PASSWORD')
+            keyAlias System.getenv('KEY_ALIAS')
+            keyPassword System.getenv('KEY_PASSWORD')
+        }
+    }
+
     buildTypes {
         release {
             shrinkResources true
             minifyEnabled true
 
-            proguardFiles 'proguard-rules.pro'
+            proguardFiles file('proguard-rules.pro')
+            signingConfig signingConfigs.release
         }
 
         debug {