fix: warnings about casing when building the Docker image

[HarikrishnanBalagopal]

Description of the change

Changed the casing of the keyword as to AS in the Dockerfile.

Related issue number

Fixes https://github.com/foundation-model-stack/fms-hf-tuning/issues/319

How to verify the PR

Build the docker image as recommended in the build/README.md

Was the PR tested

• [x] I have added >=1 unit test(s) for every new method I have added.
• [x] I have ensured all unit tests pass

[kmehant]

@jbusche Can you review and merge it?

cc: @anhuong @Ssukriti