We present an ultrastable laser system based on miniature core components: a self-injection-locked laser comprising a distributed-feedback laser diode and an integrated microring resonator, together with a 3-ml vacuum-gap Fabry-Perot cavity. By carefully addressing technical noise sources, in particular residual amplitude modulation, we demonstrate a thermal-noise-limited fractional frequency instability of 5·10⁻¹⁵ for averaging times between 1 and 10 s, which is among the lowest reported for compact cavity-stabilized lasers. This performance is achieved with conventional multilayer mirrors and a moderate cavity finesse of 10⁵. Such stability of the output frequency, combined with the small size of the core components and a possibility to generate a Kerr frequency comb in the very same microring resonator, provides a path towards fully integrated photonic frequency references for transportable optical clocks and low-noise microwave synthesis.