ci: build "master" container image and publish it to ghcr on every push

.github/workflows/build-container.yml

@@ -0,0 +1,22 @@
+name: Build Container image and publish it to GitHub Container Registry
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  publish-docker-image:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Login to GitHub Container Registry
+      uses: docker/login-action@v1
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+    - name: Build and push Docker image to the GHCR
+      run: |
+             docker build . --tag ghcr.io/gbdev/rgbds:master
+             docker push ghcr.io/gbdev/rgbds:master