You forgot the mass-energy equivalence. Photon has no rest mass, but it's energy is equivalent to some mass.
Photon's stress–energy tensor causes similar effect on gravitational field as tiny amount of mass equivalent to the energy of a photon.
Photon's stress–energy tensor causes similar effect on gravitational field as tiny amount of mass equivalent to the energy of a photon.
Photon has momentum, zero rest mass. Energy mass equivalence says that you can annihilate mass to create a massless particle. This is true for all the observers.
Relativistic mass is a perceived mass dependent on the observer.