There is no reason at all to think that he used doping in this or any other race. As there is such a major problem with illegal drug use in professional cycling, and especially in the Tour de France, it is only natural that people should wonder about this and so of course the question is being asked; but there has never been any scandal linked to Chris Froome and he has passed all the usual tests.
It was mainly the case of Lance Armstrong that made the general public aware of the doping problems in cycling. Armstrong managed to go undetected for many years, and what aroused suspicion in the end was not a test result but his achievements, which seemed to go beyond the limits of what was possible without drugs. This can't be said of Froome, impressive though his feats are - he is also an Olympic medallist.