build: Don't fail the build if DockerHub login fails (#36100)

This iterates on https://github.com/openedx/edx-platform/pull/36089, which
logged us into DockerHub during unit tests in order to reduce how often
DockerHub rate-limits us.

Forks will fail to log into DockerHub unless the fork owner configures 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

@@ -73,8 +73,12 @@ 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/login-action@v3.3.0
+        continue-on-error: true
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_PASSWORD }}