[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>

.devcontainer/devcontainer.json

@@ -3,6 +3,14 @@
   "image": "git.haxx.space/sjg/trx-rs/sdk:latest",
   "workspaceFolder": "/work",
   "workspaceMount": "source=${localWorkspaceFolder},target=/work,type=bind",
+  "mounts": [
+    "source=trx-rs-sccache,target=/sccache,type=volume"
+  ],
+  "containerEnv": {
+    "RUSTC_WRAPPER": "sccache",
+    "CARGO_INCREMENTAL": "0",
+    "SCCACHE_DIR": "/sccache"
+  },
   "customizations": {
     "vscode": {
       "extensions": [