Fix codestyle and docs jobs in travis

[eviljeff]

After mozilla/addons-server#10592 the codestyle job isn't actually doing a flake8 check because the make invocation is missing the -f to specify the correct Makefile. (It didn't need one before because the recipe was in the primary Makefile before, and now it's in Makefile-docker).
Also the docs job is broken (for an unknown period of time) for the opposite reason - the recipe is in Makefile, not Makefile-docker.

We should fix the things. Bonus for simplifying and making it so we don't have to do make -f ... everywhere in tox.ini

[EnTeQuAk]

oh man, don't trust successful outputs 👀

[AlexandraMoga]

Please add STR or the label qa:not needed.
Thanks