ci: Use https:// rather than ssh for clone

.github/workflows/pull_request.yml

@@ -0,0 +1,25 @@
+name: pull_request
+
+on: [pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Unshallow
+        run: git fetch --prune --unshallow && git submodule update --init --recursive
+      - uses: docker/setup-buildx-action@v1
+      - name: Docker login
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+      - name: Just build (no push)
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          push: false
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "src/indexer/backward-cpp"]
-	path = src/indexer/backward-cpp
-	url = git@github.com:bombela/backward-cpp.git
+    path = src/indexer/backward-cpp
+    url = https://github.com/bombela/backward-cpp.git