fix(docker): make keywords uppercase

[tessus]

Description

The 'as' keyword should match the case of the 'from' keyword

FromAsCasing: 'as' and 'FROM' keywords' casing do not match
More info: https://docs.docker.com/go/dockerfile/rule/from-as-casing/

Motivation and Context

gh action produces a warning. see here: https://github.com/orhun/rustypaste/pull/343/files#diff-dd2c0eb6ea5cfc6c4bd4eac30934e2d5746747af48fef6da689e85b752f39557

How Has This Been Tested?

• waiting for the CI to complete.

Changelog Entry

### Fixed

- Dockerfile: The 'as' keyword should match the case of the 'from' keyword

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.