Fixes #18075: Dockerfile lint warning

[kuma0128]

Describe your changes:

This is my first PR to OSS. Fixes #18075

I have fixed the warnings mentioned below.

2 warnings found (use docker --debug to expand):

• LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format
• FromAsCasing: 'as' and 'FROM' keywords' casing do not match

Type of change:

• [ ] Bug fix
• [x] Improvement
• [ ] New feature
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My PR title is Fixes <issue-number>: <short explanation>
• [ ] I have commented on my code, particularly in hard-to-understand areas.
• [ ] For JSON Schema changes: I updated the migration scripts or explained why it is not needed.