Replace the nginx + custom-build rate-limiting example with a web
service + Redis sidecar using two public images. The previous example
referenced an nginx.conf and Dockerfile that weren't shown, so readers
couldn't run it as written.

fly deploy needs at least one Dockerfile, buildpack, or image in
fly.toml to build from — a compose file with only pre-built images
fails with "app does not have a Dockerfile or buildpacks configured."
Add a minimal Dockerfile and switch the web service to build: . so
the example is runnable as written. Also tighten the limitation
wording from "only one" to "exactly one."

The example mapped container port 80 to host port 8080 in compose.yml
and set internal_port = 8080 in fly.toml, but nginxdemos/hello listens
on 80, so the Fly proxy forwarded traffic to a port nothing was
listening on. Align everything on port 80.

The [build.compose] block syntax was introduced in commit ee0ccdb
(merged 2025-07-07 19:42 UTC), which first shipped in v0.3.152
(released 2025-07-08). v0.3.149-v0.3.151 only accept the older
string-typed compose = "file.yml" field under [build] and reject
the [build.compose] block with an unmarshal error. Verified by
running fly config validate against each version.