Add CI workflow to automatically build and push new Docker images

.github/workflows/main.yml

@@ -0,0 +1,68 @@
+name: Build Docker images
+
+on:
+  # Allow manual runs.
+  workflow_dispatch:
+  # Also run on updates to this repo.
+  push:
+    branches:
+      - main
+    paths-ignore:
+      - '**/*.md'
+  # Run on PRs except for documentation.
+  pull_request:
+    paths-ignore:
+      - '**/*.md'
+  
+env:
+  IMAGE_NAME: starbound-server
+  # This doesn't apply to Starbound but to build a multi-arch Docker image you can
+  # set multiple platforms like this:
+  # PLATFORMS: linux/arm/v6,linux/arm/v7,linux/arm64/v8,linux/amd64
+  PLATFORMS: linux/amd64
+  # Only push the image on merges to main
+  PUSH_IMAGE: ${{ github.ref == 'refs/heads/master' }}
+
+jobs:
+
+  build-image:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set datestamp for image tagging
+        run: |
+          echo DATESTAMP=$(date +%Y.%m.%d) >> $GITHUB_ENV
+
+      - name: Docker Setup QEMU
+        uses: docker/setup-qemu-action@v1
+
+      - name: Docker Setup Buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Login to DockerHub
+        if: ${{ env.PUSH_IMAGE == 'true' }}
+        uses: docker/login-action@v1 
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Login to GitHub Container Registry
+        if: ${{ env.PUSH_IMAGE == 'true' }}
+        uses: docker/login-action@v1 
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GHCR_TOKEN }}
+
+      - name: Build and push ${{ env.IMAGE_NAME }} Docker image
+        uses: docker/build-push-action@v2
+        with:
+          tags: |
+            ${{ secrets.DOCKERHUB_USERNAME || 'nobody' }}/${{ env.IMAGE_NAME }}:latest
+            ghcr.io/${{ github.repository_owner }}/${{ env.IMAGE_NAME }}:latest
+            ${{ secrets.DOCKERHUB_USERNAME || 'nobody' }}/${{ env.IMAGE_NAME }}:v${{ env.DATESTAMP }}
+            ghcr.io/${{ github.repository_owner }}/${{ env.IMAGE_NAME }}:v${{ env.DATESTAMP }}
+          platforms: ${{ env.PLATFORMS }}
+          push: ${{ env.PUSH_IMAGE }}