ci: added merge-variants workflow

.github/workflows/merge-variants.yml

@@ -0,0 +1,16 @@
+name: Merge image variants
+on:
+  push:
+    branches:
+      - master
+jobs:
+  merge:
+    if: github.repository == 'itzg/docker-minecraft-server'
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v2.3.4
+      - run: |
+          git config --add user.name "${GITHUB_ACTOR:-Github Actions}"
+          git config --add user.email "actions@github.com"
+          ./docker-versions.create.sh