Add basic GitHub workflows builds (#1578)

* add easy target for backend build

* add reusable backend build workflow

* add reusable frontend build workflow

* add full build workflow

* add some comments

.github/workflows/backend-pr.yaml

@@ -0,0 +1,104 @@
+name: Backend PR
+
+on:
+  workflow_call:
+  workflow_dispatch:
+  pull_request:
+    branches:
+    - master
+    paths:
+    - backend
+    - libs
+
+jobs:
+  libs:
+    name: Build libs
+    strategy:
+      matrix:
+        target: [amd64, arm64]
+        include:
+          - target: amd64
+            snapshot_command: ./build-v8-snapshot.sh
+            artifact_name: js_snapshot
+            artifact_path: libs/js_engine/src/artifacts/JS_SNAPSHOT.bin
+          - target: arm64
+            snapshot_command: ./build-arm-v8-snapshot.sh
+            artifact_name: arm_js_snapshot
+            artifact_path: libs/js_engine/src/artifacts/ARM_JS_SNAPSHOT.bin
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+      with:
+        submodules: recursive
+        
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v1
+      
+    - name: Set up Docker Buildx        
+      uses: docker/setup-buildx-action@v1
+    
+    - uses: actions/cache@v3
+      with:
+        path: |
+          ~/.cargo/bin/
+          ~/.cargo/registry/index/
+          ~/.cargo/registry/cache/
+          ~/.cargo/git/db/
+          target/
+        key: ${{ runner.os }}-cargo-libs-${{ matrix.target }}-${{ hashFiles('libs/Cargo.lock') }}
+
+    - name: Build v8 snapshot
+      run: ${{ matrix.snapshot_command }}
+      working-directory: libs
+
+    - uses: actions/upload-artifact@v3
+      with:
+        name: ${{ matrix.artifact_name }}
+        path: ${{ matrix.artifact_path }}
+
+  backend:
+    name: Build backend
+    runs-on: ubuntu-latest
+    needs: libs
+    steps:
+    - uses: actions/checkout@v3
+      with:
+        submodules: recursive
+    
+    - name: Download arm_js_snapshot artifact
+      uses: actions/download-artifact@v3
+      with:
+        name: arm_js_snapshot
+        path: libs/js_engine/src/artifacts/
+        
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v1
+      
+    - name: Set up Docker Buildx        
+      uses: docker/setup-buildx-action@v1
+
+    - uses: actions-rs/toolchain@v1
+      with:
+        toolchain: stable
+        override: true
+    
+    - uses: actions/cache@v3
+      with:
+        path: |
+          ~/.cargo/bin/
+          ~/.cargo/registry/index/
+          ~/.cargo/registry/cache/
+          ~/.cargo/git/db/
+          target/
+        key: ${{ runner.os }}-cargo-backend-${{ hashFiles('backend/Cargo.lock') }}
+
+    - name: Build backend
+      run: make backend
+
+    - name: 'Tar files to preserve file permissions'
+      run: tar -cvf backend.tar backend/target/aarch64-unknown-linux-gnu/release/embassy*
+
+    - uses: actions/upload-artifact@v3
+      with:
+        name: backend
+        path: backend.tar