The "special" value of `ROOT` will serve requests on `/`. Any other
value should serve on `/$BASE_URL` (as long as there are no slashes
in `BASE_URL` at least).
The default has been set to `plantuml` to better match documentation
in the README.md file.
Re #80, may help with #79.
Although sequence diagrams render fine without, any(?) other diagram
type requires the `GRAPHVIZ_DOT` environment variable to be set for
the server to produce usable output.
In order to use this functionality, you need to pass `--build-arg`
options for the `http_proxy`, `https_proxy` and `no_proxy` values
that are suitable for *your* build environment.
If not behind a proxy, simply don't pass any `*_proxy` build args.
