ci: add container publishing workflow

.github/workflows/publish.yaml

@@ -0,0 +1,22 @@
+name: Publish container
+on:
+  push:
+    branches:
+      - master
+  workflow_dispatch:
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Login to Docker Hub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_HUB_TOKEN }}
+          password: ${{ secrets.DOCKER_HUB_TOKEN }}
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Publish container to Docker Hub
+        run: docker buildx build --platform linux/amd64,linux/arm64,linux/arm/v7,linux/arm/v6 --tag "mtoohey/standardnotes-extensions:$(git log -1 --format="%H" | cut -c -6)" --tag mtoohey/standardnotes-extensions:latest --push .