You can store the signing information setting environment variables.
These values can be accessed with System.getenv("<VAR-NAME>")
In your build.gradle
you can define:
signingConfigs {
release {
storeFile file(System.getenv("KEYSTORE"))
storePassword System.getenv("KEYSTORE_PASSWORD")
keyAlias System.getenv("KEY_ALIAS")
keyPassword System.getenv("KEY_PASSWORD")
}
}