Make build stage names lowercase
When using the new Docker BuildKit (since Docker 18.06) instead of the legacy build I get the following error message:
failed to solve with frontend dockerfile.v0: failed to create LLB definition: failed to parse stage name "BASE_IMAGE": invalid reference format: repository name must be lowercase
This seems to happen because the all caps BASE_IMAGE
name is used as an image name in a FROM
statement. As image names (and other name components) have to be lowercase anyway (see regexp.go on github.com), I propose that the names are changed to all lowercase.