[chore](trx-rs): add sccache compilation cache

Bake sccache into the SDK image and enable it via RUSTC_WRAPPER in CI and
the devcontainer (not repo-wide, so non-SDK builds are unaffected).

- container/Containerfile: install the sccache musl binary.
- ci.yml: RUSTC_WRAPPER=sccache, CARGO_INCREMENTAL=0, SCCACHE_DIR=/sccache,
  cache size cap, plus a `sccache --show-stats` step per job.
- runner-config.example.yaml: bind-mount /var/cache/sccache into job
  containers so the cache persists across runs and is shared between jobs.
- .devcontainer: enable sccache with a named cache volume.

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

container/README.md

@@ -54,6 +54,22 @@ The `reuse` job stays on the upstream `fsfe/reuse-action` (a Docker action the
 Docker executor launches as a sibling container) — nothing REUSE-related is
 baked into the SDK.
 
+## Compilation cache (sccache)
+
+The SDK image ships [`sccache`](https://github.com/mozilla/sccache). It is
+enabled via `RUSTC_WRAPPER=sccache` in CI and the devcontainer (not repo-wide,
+so plain `cargo` builds outside the SDK are unaffected).
+
+- **CI** persists the cache on the runner host — create the dir once:
+  `mkdir -p /var/cache/sccache`. It is bind-mounted into each job container at
+  `/sccache` (see `runner-config.example.yaml`), so cache survives across runs
+  and is shared between the lint/test jobs and both projects.
+- **Devcontainer** uses a named volume (`trx-rs-sccache`).
+- Check effectiveness with `sccache --show-stats` (the CI jobs print it).
+
+`CARGO_INCREMENTAL=0` is set wherever sccache is on, since sccache cannot cache
+incremental artifacts.
+
 ## CI runner (Alpine / OpenRC)
 
 The runner uses the **Docker executor** (not the host executor): per-job