Fix warnings when building Docker image

 3 warnings found (use --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 1)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 8)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 23)