CI: use GitHub Actions to build images

.github/workflows/ci.yml

@@ -0,0 +1,24 @@
+name: 'Citra Docker Image CI'
+on:
+  push:
+    branches: ["master"]
+  pull_request:
+    branches: ["master"]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        image: ["linux-clang-format", "linux-flatpak", "linux-fresh", "linux-frozen", "linux-mingw", "linux-transifex"]
+    
+    steps:
+    - uses: actions/checkout@v2
+    - uses: docker/setup-buildx-action@v1
+      name: Setup Docker BuildX system
+    - uses: docker/build-push-action@v2
+      name: Build image
+      with:
+        push: false
+        context: ${{ matrix.image }}
+        tags: citraemu/build-environments:${{ matrix.image }}