.github: Add build workflow

Author: the-maldridge

.github/workflows/docker.yml

@@ -0,0 +1,29 @@
+name: docker
+
+on:
+  push:
+    tags:
+      - '*'
+
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Login to GCHR
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: void-robot
+          password: ${{ secrets.CR_PAT }}
+      - name: Set version
+        run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
+      - name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          tags: "ghcr.io/${{ github.repository }}:${{ env.RELEASE_VERSION }}"