[Azure] Fix docker login on Azure

[Michaelvll]

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)

  resources:
    cloud: azure
    image_id: docker:michaelvll/skypilot-test:v4
  
  
  envs:
    SKYPILOT_DOCKER_USERNAME: michaelvll
    SKYPILOT_DOCKER_PASSWORD: "xxxx"
    SKYPILOT_DOCKER_SERVER: docker.io
  

• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[cblmemo]

Thanks for identifying and fixing this @Michaelvll ! LGTM :))