dockerfile: change output when loading Dockerfile
dockerfile: change output when loading Dockerfile

Previously, the output said "load Dockerfile from " + filename when the
file is not named Dockerfile. With this patch, it is changed to always
show "load build definition from " + filename.

This is because `# syntax = ...` can be put in non-Dockerfiles.

Signed-off-by: Tibor Vass <3f1326dff4d85ad4ec938fb33f64575ace96db26@docker.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc