release signed apk using gradle

build.gradle

android {
     signingConfigs {
             release {
                 storeFile file("/path/to/keystore")
                 storePassword "storepassword"
                 keyAlias "alias"
                 keyPassword "keypassword"
             }
        }
     buildTypes {
            release {
                signingConfig signingConfigs.release
            }
        }
    }