build: Don't fail the build if DockerHub login fails

Iterating on openedx#36089

Forks will fail to log into DockerHub unless the fork owner
configured their own DockerHub creds. This PR is an attempt
to make it so that unit tests don't fail when DockerHub login
fails.

.github/workflows/unit-tests.yml

@@ -73,13 +73,17 @@ jobs:
         run: |
           sudo apt-get update && sudo apt-get install libmysqlclient-dev libxmlsec1-dev lynx
 
-      - name: Login to Docker Hub
+      # Try to log into DockerHub so that we're less likely to be rate-limited when pulling certain images.
+      # This will fail on any edx-platform fork which doesn't explicitly define its own DockerHub creds.
+      # That's OK--if we fail to log in, we'll proceed anonymously, and hope we don't get rate-limited.
+      - name: Try to log into Docker Hub
         uses: docker/[email protected]
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_PASSWORD }}
 
       - name: Start MongoDB
+        if: always()  # Continue even if the previous step (log into DockerHub) failed.
         uses: supercharge/[email protected]
         with:
           mongodb-version: ${{ matrix.mongo-version }}