Fix checked out branch for PR docker image build workflow (feast-dev#…

…2018)

Signed-off-by: Tsotne Tabidze <tsotne@tecton.ai>

.github/workflows/pr_integration_tests.yml

@@ -15,6 +15,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          # pull_request_target runs the workflow in the context of the base repo
+          # as such actions/checkout needs to be explicit configured to retrieve
+          # code from the PR.
+          ref: refs/pull/${{ github.event.pull_request.number }}/merge
+          submodules: recursive
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v1
       - name: Set up Docker Buildx