[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/runner-config.example.yaml

@@ -27,7 +27,9 @@ cache:
 
 container:
   # Cap every job container's CPU so CI stays within the 2-thread budget even
-  # if capacity is raised later.
-  options: "--cpus=2"
+  # if capacity is raised later. The -v mount persists the sccache cache on the
+  # host (create it first: `mkdir -p /var/cache/sccache`), matching SCCACHE_DIR
+  # in the workflow.
+  options: "--cpus=2 -v /var/cache/sccache:/sccache"
   # Reuse the host VM's Docker network for the built-in cache/artifact server.
   network: "host"