fix: inconsistent casing of FROM and AS in Dockerfile

[ShotaroMuraoka]

Proposed changes

This PR fixes an inconsistency in the casing of FROM and AS in the Dockerfile.
Currently, FROM is uppercase while as is lowercase, which can trigger warnings in certain build environments.
To maintain consistency and follow Dockerfile best practices, AS is now capitalized.

Warning message before the fix:

 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 5)

There is no functional change in this PR; it is purely a stylistic improvement.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc.
• I have added tests (when possible) that prove my fix is effective or that my feature works.
• I have checked that all unit tests pass after adding my changes.
• I have updated necessary documentation.
• I have rebased my branch onto master.
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork.

[bavshin-f5]

Thanks!