Dockerfile: set `HOMEBREW_*` variables

This matches what `brew shellenv` prints and make shell completions work
in containers (and codespaces). Evaluating `brew shellenv` does not work
because `brew shellenv` detects that `PATH` is already correctly set and
thus exits without printing anything.

I don't think `MANPATH` and `INFOPATH` (which are also printed by
`brew shellenv`) are needed in the Dockerfile because `man` and `info`
are not installed in the image.