Add GitHub workflow to rebuild SDL binaries for Linux

.github/workflows/build.yml

@@ -0,0 +1,30 @@
+name: build-linux
+on: [push]
+#  schedule:
+#    - cron: '0 0 * * *'
+jobs:
+  build-linux:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build i386 binaries
+      run: |
+        mkdir /tmp/i386
+        cd linux-build
+        docker build -t sdl2-i386 -f Dockerfile.i386 .
+        docker run -v `pwd`/compile:/entry -v /tmp/i386:/io sdl2-i386
+        cp /tmp/i386/lib/libSDL2-2.0.so.0 ../native/linux-x86
+    - name: Build amd64 binaries
+      run: |
+        mkdir /tmp/amd64
+        cd linux-build
+        docker build -t sdl2-amd64 -f Dockerfile.amd64 .
+        docker run -v `pwd`/compile:/entry -v /tmp/amd64:/io sdl2-amd64
+        cp /tmp/amd64/lib/libSDL2-2.0.so.0 ../native/linux-x64
+    - name: Create pull request
+      uses: peter-evans/create-pull-request@v2
+      with:
+        commit-message: 'Update Linux SDL binaries'
+        body: This PR has been auto-generated to update the Linux SDL binaries.
+        branch: update-linux-binaries
+