It is perplexing for me as someone who likes dog to comprehend the etymology of this phrase (if any). I wouldn’t associate cannibalism with dogs. Most of the dogs I have known or seen are the cute friendly cooperative thing that can even evoke a warm fuzzy feeling inside. If anything, I would say dog symbolises loyalty, community, assistance, and even cooperation. It seems incongruent that it is used to describe a world where success must come at someone else’s expense.
Perhaps the phrase is from back when dogs were wild and had to compete for survival before some became domesticated as house pets. Or perhaps it was coined to express the idea that an environment is so competitive that the metaphorical dog (which would otherwise be cooperative) have to ‘eat’ another metaphorical dog in order to survive.
Instead of using man’s best friend as scapegoat, I’d prefer to use the phrase “every man for himself”.