[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>

.gitea/workflows/ci.yml

@@ -16,6 +16,13 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
+  # sccache: shared compilation cache persisted on the runner host (see the
+  # -v mount in runner-config.example.yaml). CARGO_INCREMENTAL=0 because
+  # sccache cannot cache incremental artifacts.
+  RUSTC_WRAPPER: sccache
+  CARGO_INCREMENTAL: "0"
+  SCCACHE_DIR: /sccache
+  SCCACHE_CACHE_SIZE: "20G"
 
 jobs:
   lint:
@@ -27,6 +34,9 @@ jobs:
         run: cargo fmt --all -- --check
       - name: clippy
         run: cargo clippy --workspace --all-targets --all-features -- -D warnings
+      - name: sccache stats
+        if: always()
+        run: sccache --show-stats
 
   test:
     runs-on: ubuntu-latest
@@ -37,6 +47,9 @@ jobs:
         run: cargo build --workspace --all-targets --locked
       - name: Test
         run: cargo test --workspace --locked
+      - name: sccache stats
+        if: always()
+        run: sccache --show-stats
 
   frontend:
     runs-on: ubuntu-latest