chore: fix docker build warnings

[harryzcy]

Proposed changes

Fix the following two warnings:

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

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[ferrarimarco]

Thanks for this PR!

There's also a --check option. It might be interesting to add it as a test. We can do that in another PR.