chore: add syntax parser directive to Dockerfile

[Marukome0743]

Description

Add syntax parser directive to the first line of the Dockerfile.

Motivation and Context

To declare the Dockerfile syntax version to use for the build.
If unspecified, BuildKit uses a bundled version of the Dockerfile frontend.

Usage examples

# syntax=docker/dockerfile:1
FROM brainpower/node-cubicle

WORKDIR /root/repo

ADD . ./
RUN yarn
RUN yarn build

How Has This Been Tested?

I have built using changed Dockerfile.

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)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

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