.github/workflows: add build-openbios-builder.yml action

This is a GitHub manual action that generates a container image from
docker/Dockerfile.builder and pushes the result to
ghcr.io/openbios/openbios-builder:master for public use.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

.github/workflows/build-openbios-builder.yml

@@ -0,0 +1,42 @@
+name: Build openbios-builder container
+
+on:
+  # Run manually from Actions tab
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}-builder
+
+jobs:
+  build-x86_64:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Log in to the Container registry
+        uses: docker/login-action@f054a8b539a109f9f41c372932f1ae047eff08c9
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@98669ae865ea3cffbcbaa878cf57c20bbf1c6c38
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@ad44023a93711e3deb337508980b4b5e9bcdc5dc
+        with:
+          context: .
+          file: docker/Dockerfile.builder
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}