sphinx-doc/sphinx#11094

`extlinks` config expects a formatting slug in the caption portion of
its config. Without it, Sphinx 6.x throws:

    Exception occurred:
      File "/path/to/site-packages/sphinx/ext/extlinks.py", line 103, in role
        title = caption % part
    TypeError: not all arguments converted during string formatting

When creating a Dockerfile from redirecting debug output, `repo2docker`
sees the initially-empty `Dockerfile` and that influences its output
(since `Dockerfile` is empty, `repo2docker` outputs nothing).

…ped past me

I did not see the "Can I use repo2docker to bootstrap my own
Dockerfile?" FAQ the first time through. I've linked it from the part
that I _did_ see so hopefully the next person through (maybe me in 6
months) won't have the same misunderstanding!