AzureML Compute Job failed to start

Unable to use dockerfile to create an Environment in the `explain-model-on-amlcompute.ipynb` notebook.

The following is the code

```
from azureml.core import Workspace, Environment
myenv = Environment(name="rapidsenv")

myenv.register(workspace=ws)
myenv.docker.enabled=True

dockerfile = r"""
FROM rapidsai/rapidsai-cloud-ml:0.17-cuda11.0-base-ubuntu18.04-py3.8
RUN echo "success" # I'd like to install different packages here
"""
myenv.docker.base_image = None
myenv.docker.base_dockerfile = dockerfile
myenv.python.user_managed_dependencies=True

from azureml.core import Run
from azureml.core import ScriptRunConfig

src = ScriptRunConfig(source_directory=project_folder,
script='train_explain.py',
compute_target=gpu_cluster,
environment=myenv)
run = experiment.submit(config=src)
run
```

This is what shows up on the Portal.

```
AzureMLCompute job failed.
JobContainerConfigFailed: Container configuration failed unexpectedly
JobContainerConfigFailed: Container configuration failed unexpectedly
err: Docker exec failure when attempting to execute 'containerSetup' task. Error: exit status 126. Indicates a command was found but not executable, likely due to permission issues or missing dependencies.
Reason: Docker exec failure when attempting to execute 'containerSetup' task. Error: exit status 126. Indicates a command was found but not executable, likely due to permission issues or missing dependencies.
Info: Failed to prepare an environment for the job execution: Job environment preparation failed on 10.0.0.4 with err exit status 1.
```

How do I resolve this issue?

從 explain-model-on-amlcompute.ipynb 開始，針對 GPU compute target 執行提供的 Environment 和 ScriptRunConfig 範例。使用 Portal 的 JobContainerConfigFailed 和 containerSetup exit-status-126 訊息追蹤環境準備程序；當 notebook 工作使用以 Dockerfile 為基礎的環境成功啟動時，即表示完成。

由索引模型根據 Issue 內容生成。