Users may inadvertently set the builder to 'dockerfile' and have build errors when there is no Dockerfile. This change makes it clear that the dockerfile _must_ exist in order for the build to succeed. Closes #7181
Users may inadvertently set the builder to 'dockerfile' and have build errors when there is no Dockerfile. This change makes it clear that the dockerfile _must_ exist in order for the build to succeed. Closes #7181