CI: Cache prover Docker build

Problem: building the prover from scratch takes ages.

Solution: cache the generated Docker image and reuse it for subsequent
builds. This caching solution makes the build closer to a local dev
environment where the prover has already been built once.

.github/workflows/tests.yml

@@ -21,13 +21,15 @@ jobs:
         uses: actions/checkout@v3
         with:
           submodules: recursive
+
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
           toolchain: 1.73
           override: true
           components: rustfmt, clippy
+
       - name: Set up cargo cache
         uses: actions/cache@v3
         continue-on-error: false
@@ -40,9 +42,33 @@ jobs:
             target/
           key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
           restore-keys: ${{ runner.os }}-cargo-
+
+      - name: Log in to Github container registry
+        run: echo "${{ secrets.GITHUB_TOKEN }}" | docker login ghcr.io -u ${{ github.actor }} --password-stdin
+
+      - name: Set cache image environment variables
+        run: |
+          # Uppercase characters are not allowed in Docker tags
+          cache_image=$(echo ghcr.io/${GITHUB_REPOSITORY}/build-cache | tr '[:upper:]' '[:lower:]')
+          echo "STONE_PROVER_DOCKER_CACHE=$(echo ${cache_image})" >> $GITHUB_ENV
+
+
+      - name: Download Docker cache image (if available)
+        run: docker pull ghcr.io/$GITHUB_REPOSITORY/build-cache || true
+
       - name: Build
         run: cargo build --verbose
+
       - name: Lint with Clippy
         run: cargo clippy -- -D warnings
+
       - name: Run tests
         run: cargo test --verbose
+
+      - name: Push the image to the cache
+        # It's not possible to push packages from fork PRs.
+        if: github.event.pull_request.head.repo.full_name == github.repository
+        run: |
+          docker tag stone-prover-build:latest ${STONE_PROVER_DOCKER_CACHE}
+          docker push ${STONE_PROVER_DOCKER_CACHE}
+

stone-prover/build.rs

@@ -58,6 +58,20 @@ fn copy_prover_files_from_container(
     Ok(())
 }
 
+fn make_docker_build_command(repo_dir: &Path, image_name: &str) -> String {
+    let mut docker_build_command = format!(
+        "docker build -t {image_name} {}",
+        repo_dir.to_string_lossy()
+    );
+
+    // Check if a cache image exists. Used by the CI/CD pipeline.
+    if let Ok(build_cache) = std::env::var("STONE_PROVER_DOCKER_CACHE") {
+        docker_build_command.push_str(&format!(" --build-from {build_cache}"));
+    }
+
+    docker_build_command
+}
+
 /// Build the Stone Prover and copy binaries to `output_dir`.
 ///
 /// The prover repository contains a Dockerfile to build the prover. This function:
@@ -68,10 +82,7 @@ fn copy_prover_files_from_container(
 fn build_stone_prover(repo_dir: &Path, output_dir: &Path) {
     // Build the Stone Prover build Docker image
     let image_name = "stone-prover-build:latest";
-    let docker_build_command = format!(
-        "docker build -t {image_name} {}",
-        repo_dir.to_string_lossy()
-    );
+    let docker_build_command = make_docker_build_command(repo_dir, image_name);
     run_command(&docker_build_command).expect("Failed to build Stone Prover using Dockerfile");
 
     // Run a container based on the Docker image