fix(ci): add cross Docker image build step to build-and-publish workflow

The musl and aarch64 cross builds were failing with "Unable to find
image 'wallhack-cross:...' locally" because the custom Docker image
was never built before running cross. Add setup-buildx + build-push
steps (gated on matrix.custom_image) matching what pr.yml does.
Shares the GHA Docker layer cache via scope=cross-musl.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: maxholman

.github/workflows/build-and-publish.yml

@@ -35,6 +35,7 @@ jobs:
             os: ubuntu-latest
             artifact: wallhack-linux-x64-musl
             cross: true
+            custom_image: true
           - target: aarch64-unknown-linux-gnu
             os: ubuntu-latest
             artifact: wallhack-linux-arm64
@@ -91,6 +92,20 @@ jobs:
           gh release download "$CROSS_VERSION" --repo cross-rs/cross --pattern 'cross-x86_64-unknown-linux-musl.tar.gz' -D /tmp
           tar xz -C ~/.cargo/bin cross < /tmp/cross-x86_64-unknown-linux-musl.tar.gz
 
+      - uses: docker/setup-buildx-action@v4
+        if: matrix.custom_image
+
+      - name: Build and cache cross Docker image
+        if: matrix.custom_image
+        uses: docker/build-push-action@v7
+        with:
+          context: .
+          file: Dockerfile.cross-musl
+          tags: wallhack-cross:${{ matrix.target }}
+          load: true
+          cache-from: type=gha,scope=cross-musl
+          cache-to: type=gha,mode=max,scope=cross-musl
+
       - name: Build (native)
         if: ${{ !matrix.cross }}
         run: cargo build --release --target ${{ matrix.target }} -p wallhack-cli ${{ matrix.features }}