In the center, this should be the diffraction pattern from dust particles (so called "Airy disk") but not the 'onion-ring'.
Simply speaking, a point is not be projected as a sharp point but the 'airy disk' on CMOS, because of the wave character of light.
In addition, the simulation of a whole bokeh and the example similar to yours (from Hasselblad 80mm 2.8 CF T* Bokeh, Bokeh control):
A typical “onion-ring” bokeh is resulting from the aspherical elements inside your lens, which is known as a kind of processing defect resulting from aspherical elements grinding. The pattern is more uniform than the diffraction pattern.
Spherical lens is a combination of sphere, it converges the light but perfect convergence can not be achieved by ideal spherical surfaces.(spherical aberration)