Fix docker warning in `Containerfile`

[josecelano]

Relates to: https://docs.docker.com/reference/build-checks/from-as-casing/

 8 warnings found (use --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 6)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 12)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 24)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 65)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 79)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 94)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 132)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 139)