"bazel build //:envoy" fails on fresh clone of this repo

Instruction here, https://github.com/envoyproxy/envoy-filter-example?tab=readme-ov-file#building

We fail to resolve `{ENVOY_SCRDIR}`. Now, for `dev container` support, we set this at: https://github.com/envoyproxy/envoy/blob/8283565cffc7b713ef1b3a8b79c285c269e15db3/.devcontainer/devcontainer.json#L16-L18

## Error initializing container

This error is found in the `Terminal` tab.

```
Running the postCreateCommand from devcontainer.json...

[1856 ms] Start: Run in container: /bin/sh -c .devcontainer/setup.sh
.devcontainer/setup.sh: line 3: bazel/setup_clang.sh: No such file or directory
Done. Press any key to close the terminal.
```

## Error during build step

```
vscode@docker-desktop:/workspaces/envoy-filter-example$ bazel build //:envoy
WARNING: The following configs were expanded more than once: [clang-libc++, libc++, clang]. For repeatable flags, repeats are counted twice and may lead to unexpected behavior.
ERROR: /workspaces/envoy-filter-example/WORKSPACE:3:17: fetching local_repository rule //external:envoy: java.io.IOException: The repository's path is "{ENVOY_SRCDIR}" (absolute: "/workspaces/envoy-filter-example/{ENVOY_SRCDIR}") but it does not exist or is not a directory.
ERROR: Error computing the main repository mapping: no such package '@@envoy//bazel': The repository's path is "{ENVOY_SRCDIR}" (absolute: "/workspaces/envoy-filter-example/{ENVOY_SRCDIR}") but it does not exist or is not a directory.
Computing main repo mapping:
vscode@docker-desktop:/workspaces/envoy-filter-example$ echo $ENVOY_S
$ENVOY_SRCDIR $ENVOY_STDLIB
vscode@docker-desktop:/workspaces/envoy-filter-example$ echo $ENVOY_SRCDIR
/workspaces/envoy-filter-example
vscode@docker-desktop:/workspaces/envoy-filter-example$
```