bump Docker builder image to go1.12.12
bump Docker builder image to go1.12.12

hartsock/vault,hartsock/vault,hartsock/vault,hashicorp/vault,hashicorp/vault,rnaveiras/vault,hartsock/vault,quixoten/vault,hashicorp/vault,rnaveiras/vault,quixoten/vault,joelthompson/vault,joelthompson/vault,kingland/vault,rnaveiras/vault,kingland/vault,rnaveiras/vault,quixoten/vault,kingland/vault,hartsock/vault,kingland/vault,rnaveiras/vault,hashicorp/vault,hashicorp/vault,quixoten/vault,joelthompson/vault,joelthompson/vault,hashicorp/vault,kingland/vault