warnings with newer Docker syntax image

[bcressey]

After cb4d91b we no longer pin the Dockerfile syntax image. Depending on the installed version of Docker, warnings like this can now be logged.

  #1 [internal] load build definition from build.Dockerfile
  #1 transferring dockerfile: 22.46kB done
  #1 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 19)
  #1 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 197)
  #1 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 317)
  #1 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 388)
  #1 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 420)
  #1 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 464)
  #1 WARN: InvalidDefaultArgInFrom: Default value for ARG ${SDK} results in empty or invalid base image name (line 19)
  #1 WARN: SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ARG "TOKEN") (line 33)
  #1 DONE 0.0s