resolve warnings about casing in Dockerfiles (#688)

I've been seeing this build warning in logs here:

```text
 2 warnings found (use --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 37)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 70)
```

([example build link](https://github.com/rapidsai/docker/actions/runs/9773984230/job/26981297565#step:9:826))

That comes from BuildKit's build checks (https://docs.docker.com/reference/build-checks/). Details on that specific warning: https://docs.docker.com/reference/build-checks/from-as-casing/.

This fixes it, to remove a source of noise in logs (and because I agree with the suggestion that capitalizing keywords approves readability a bit).

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)

URL: #688

Dockerfile

@@ -34,7 +34,7 @@ EOF
 
 
 # Base image
-FROM rapidsai/miniforge-cuda:cuda${CUDA_VER}-base-${LINUX_VER}-py${PYTHON_VER} as base
+FROM rapidsai/miniforge-cuda:cuda${CUDA_VER}-base-${LINUX_VER}-py${PYTHON_VER} AS base
 ARG CUDA_VER
 ARG PYTHON_VER
 
@@ -67,7 +67,7 @@ CMD ["ipython"]
 
 
 # Notebooks image
-FROM base as notebooks
+FROM base AS notebooks
 
 ARG CUDA_VER
 ARG LINUX_DISTRO

context/test_notebooks.py

@@ -70,6 +70,7 @@ def test_notebook(notebook_file, executed_nb_file):
         warnings = []
         outputs = []
 
+
         # use nbconvert to run the notebook natively
         ep = ExecutePreprocessor(timeout=600, kernel_name="python3", allow_errors=True)
         task_init = timeit.default_timer()