Remove unnecessary `mkdir /app` from Dockerfile.
Remove unnecessary `mkdir /app` from Dockerfile.

`WORKDIR` creates its directory anyway, and the mkdir fails now that
we're creating `/app` in govuk-ruby-base. Thought I'd fixed all these
before merging the change to govuk-ruby-base, but turns out I forgot to
actually raise a bunch of the PRs that I'd prepared.

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend