A large class of fluids of particles interacting via ultrasoft, repulsive pair potentials crystallize into cluster crystals. Here, we employ density functional theory and computer simulations to study the behavior of a system of particles that repel each other with a $exp(-r^8)$-potential [A. J. Moreno and C. N. Likos, Phys. Rev. Lett., 2007, 99, 107801] under planar confinement. We compare the behavior for purely repulsive to that for attractive slit walls. In particular, we present the phase diagram and we show that for repulsive walls the system freezes from the middle, whereas for attractive ones crystallization sets in at the walls and proceeds to the middle. For large wall-wall-separations we find continuous growth of a fluid or crystalline layer on the wall, depending on the wall-particle interaction, which is interrupted by capillary melting or freezing close to the bulk crystallization transition. An asymptotic scaling analysis of the width of the liquid or crystalline films growing at the walls indicate complete wetting in both cases.