Why does my custom buildtype gives apk not signed error

I have the following app.gradle configuration:

buildTypes {
    release {
        minifyEnabled true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }

    debug {
        debuggable true
        // more configs here
    }

    staging {
        externalNativeBuild {
            cmake {
                cppFlags "-DDEBUG_FLAG"
            }
        }
    }
}

The staging build type gives error:

The apk for your currently selected variant is not signed.

Apparently, only buildtypes release and debug are configured with signing keys. For custom buildtypes, you have to manually set it by:

1) using the signing config of debug

staging {
    signingConfig signingConfigs.debug
    ...
}

OR

2) inherit from a buildtype that has signing key configured

staging {
    initWith debug
    ...
}

OR

3) generate a new key and create your own signing configuration

android {
    signingConfigs {
        keyStagingApp {
            keyAlias 'stagingKey'
            keyPassword 'stagingKeyPassword'
            storeFile file('../stagingKey.jks')
            storePassword 'stagingKeyPassword'
        }
    }
    ...
}

then configure the staging like so:

staging {
    signingConfig signingConfigs.keyStagingApp
    ...
}