[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": [

.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
 
   reuse:
     runs-on: ubuntu-latest

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

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

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.
+  # if capacity is raised later. The -v mount persists the sccache cache on the
-  options: "--cpus=2"
+  # 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"