Made mock-fts take its base URL from --addr when --base-url isn't given

internal/mock/README.md

@@ -40,7 +40,7 @@ Useful flags (`go run ./cmd/mock-fts --help` lists them all):
 | Flag | Default | Meaning |
 |---|---|---|
 | `--addr` | `:8080` | Listen address |
-| `--base-url` | `http://localhost:8080` | Public base URL used in `uri` and `links.next` |
+| `--base-url` | from `--addr` | Public base URL used in `uri` and `links.next`. Without it, `http://<addr>`, with `localhost` for an unspecified host (`:8080`, `0.0.0.0`, `::`). Set it when clients use another address (a proxy, a mapped container port), or with port 0 |
 | `--data-dir` | `./mock-data` | Where `releases.jsonl` is stored |
 | `--reset` | | Delete stored releases and regenerate the backfill |
 | `--backfill` | `72h` | History to generate on an empty data directory |