We demonstrate neuronlike spiking dynamics in a dissipative Bose-Hubbard dimer model, describing two coupled nonlinear Kerr cavities with a finite photon lifetime and under coherent pumping of just one cavity. Spiking dynamics appear due to the excitable nature of the system. In this context, excitable excursions in the phase space correspond to spikes in the temporal evolution of the cavity intensities, i.e., of the number of photons. In our case, excitability is mediated by the destruction of an oscillatory state in a global homoclinic bifurcation. In this type of excitability (known as type I) the period of the oscillatory state diverges when approaching the bifurcation. Beyond this point, the system exhibits excitable dynamics under the application of suitable perturbations. We have also characterized the effect that additive Gaussian noise has on the spiking dynamics, showing that the system undergoes a coherence resonance for a given value of the noise strength.