Fix warnings from building docker images

[mattkjames7]

Really minor, but we should fix these:

 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 4)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 103)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 168)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 197)
 - InvalidDefaultArgInFrom: Default value for ARG $MGBUILD_IMAGE results in empty or invalid base image name (line 4)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 13)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 52)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 112)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 155)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 177)```