[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/Containerfile

@@ -43,4 +43,13 @@ RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs \
         --component rustfmt --component clippy \
     && chmod -R a+rwX "$RUSTUP_HOME" "$CARGO_HOME"
 
+# sccache — shared compilation cache. Enabled at build time via
+# RUSTC_WRAPPER (see the CI workflow and .devcontainer), not repo-wide, so
+# non-SDK builds are unaffected. musl build is static and runs anywhere.
+ARG SCCACHE_VERSION=0.8.2
+RUN curl -fsSL "https://github.com/mozilla/sccache/releases/download/v${SCCACHE_VERSION}/sccache-v${SCCACHE_VERSION}-x86_64-unknown-linux-musl.tar.gz" \
+      | tar -xz -C /tmp \
+    && install -m755 "/tmp/sccache-v${SCCACHE_VERSION}-x86_64-unknown-linux-musl/sccache" /usr/local/bin/sccache \
+    && rm -rf /tmp/sccache-*
+
 WORKDIR /work