Split buildx action into main and dev builds

Since Docker Hub no longer allows automated builds for free tier users,
the build process for new images needs to be moved to GitHub Actions.
The existing buildx workflow has worked pretty well for the most part,
but was only enabled for the develop branch and only pushed the
buildx-experimental tag. This addition allows pushes to the main branch
to build updates for the "latest" tag as well, which is more commonly
used I think.

.github/workflows/buildx-main.yml

@@ -0,0 +1,28 @@
+name: buildx
+
+on:
+  push:
+    branches: main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout code
+        uses: actions/checkout@v2
+      - name: install buildx
+        id: buildx
+        uses: crazy-max/ghaction-docker-buildx@v1
+        with:
+          version: latest
+      - name: log in to docker hub
+        run: |
+          echo "${{ secrets.DOCKER_PASSWORD }}" | \
+            docker login -u "${{ secrets.DOCKER_USERNAME }}" --password-stdin
+      - name: build and push the image
+        run: |
+          docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
+          docker buildx ls
+          docker buildx build --push \
+            --tag benbusby/whoogle-search:latest \
+            --platform linux/amd64,linux/arm/v7,linux/arm64 .