perf(ci): skip Docker build for Linux cargo check when inputs unchanged

Add hash-based cache check using actions/cache@v4 before Docker
builder setup in test.yml. Hash covers Cargo manifests, lockfile,
Rust source, cargo config, and the Dockerfile. On cache hit, the
entire Docker setup/build/teardown is skipped, eliminating ~65s of
orchestration overhead when no Rust source has changed.

Expected: ~75s -> ~5s on cache hit (93% reduction).
Cache miss path is unchanged — full Docker build runs as before.
release.yml is unaffected (always builds artifacts).

Author: pythoninthegrass

.github/workflows/test.yml

@@ -123,20 +123,41 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
+      # Linux: skip Docker build when cargo-check inputs haven't changed.
+      # Hash covers everything the Dockerfile check target copies:
+      # Cargo manifests, lockfile, Rust source, cargo config, and the
+      # Dockerfile itself.  Cache hit means cargo check already passed
+      # for these exact inputs — no need to spin up Docker at all.
+      - name: Check cargo-check cache (Linux)
+        if: runner.os == 'Linux'
+        id: cargo-check-cache
+        uses: actions/cache@v4
+        with:
+          path: .cargo-check-sentinel
+          key: linux-cargo-check-${{ hashFiles('Cargo.toml', 'Cargo.lock', 'crates/**/*.rs', 'crates/**/Cargo.toml', 'crates/**/.cargo/**', 'docker/linux/Dockerfile') }}
+
+      - name: Skip notice (Linux)
+        if: runner.os == 'Linux' && steps.cargo-check-cache.outputs.cache-hit == 'true'
+        run: echo "cargo check inputs unchanged — skipping Docker build"
+
       # Linux: containerized cargo check with Blacksmith Docker layer caching
       - name: Setup Docker builder (Linux)
-        if: runner.os == 'Linux'
+        if: runner.os == 'Linux' && steps.cargo-check-cache.outputs.cache-hit != 'true'
         uses: useblacksmith/setup-docker-builder@v1
 
       - name: Cargo check (Linux)
-        if: runner.os == 'Linux'
+        if: runner.os == 'Linux' && steps.cargo-check-cache.outputs.cache-hit != 'true'
         uses: useblacksmith/build-push-action@v2
         with:
           context: .
           file: docker/linux/Dockerfile
           target: check
           push: false
 
+      - name: Write cargo-check sentinel (Linux)
+        if: runner.os == 'Linux' && steps.cargo-check-cache.outputs.cache-hit != 'true'
+        run: date -u > .cargo-check-sentinel
+
       # macOS/Windows: bare-metal setup + cargo check
       - name: Setup Tauri build environment
         if: runner.os != 'Linux'

.gitignore

@@ -1,6 +1,7 @@
 # ETC
 *.db*
 *.sqlite
+.cargo-check-sentinel
 .playwright-mcp/
 .sisyphus/
 .task/

docs/builds.md

@@ -416,14 +416,17 @@ Linux amd64 builds use a multi-stage Dockerfile (`docker/linux/Dockerfile`) for
 
 In CI, [Blacksmith Docker layer caching](https://docs.blacksmith.sh/blacksmith-caching/docker-builds) persists all layers on NVMe-backed sticky disks. After the first run, the `deps` and `cook` layers are cached — subsequent runs only rebuild changed source code.
 
+The test workflow (`test.yml`) uses a hash-based skip optimization: before spinning up Docker, it computes a hash of all files that affect `cargo check` output (`Cargo.toml`, `Cargo.lock`, `crates/**/*.rs`, crate manifests, cargo config, and the Dockerfile). If the hash matches a previous successful run (via `actions/cache`), the entire Docker build is skipped. This eliminates ~65s of Docker orchestration overhead on runs where Rust source hasn't changed — reducing the Linux job from ~75s to ~5s on cache hit.
+
 ```yaml
-# test.yml — cargo check only
-- uses: useblacksmith/setup-docker-builder@v1
-- uses: useblacksmith/build-push-action@v2
+# test.yml — cargo check only (with hash-based skip)
+- uses: actions/cache@v4        # check if inputs changed
+- uses: useblacksmith/setup-docker-builder@v1  # skip if cache hit
+- uses: useblacksmith/build-push-action@v2     # skip if cache hit
   with:
     target: check
 
-# release.yml — full build + bundle, extract artifacts
+# release.yml — full build + bundle, extract artifacts (no skip)
 - uses: useblacksmith/setup-docker-builder@v1
 - uses: useblacksmith/build-push-action@v2
   with: