Allow preview even if no Dockefile inline and location are not defined

When using `dockerfile.inline` parameter, we would use "future"
reference of something that has not already been computed during the
preview stage.
In this case, `dockefile.location` and `dockerfile.infile` can be empty
but can be available during the processing stage.

Signed-off-by: Alexandre Nicolaie <alexandre.nicolaie@radiofrance.com>

provider/internal/context.go

@@ -127,9 +127,12 @@ func (bc *BuildContext) validate(preview bool, d *Dockerfile) (*Dockerfile, *Con
 		return d, c, newCheckFailure(err, "context.location")
 	}
 
-	if d.Location == "" && d.Inline == "" {
+	if d.Location == "" && d.Inline == "" && !preview {
 		// If a Dockerfile wasn't provided and our context is on-disk, then
-		// set our Dockerfile to a default of <PATH>/Dockerfile.
+		// set our Dockerfile to a default of <PATH>/Dockerfile. However, if
+		// we're in preview mode, we don't want to do this because we don't
+		// know if the inline Dockerfile parameter contains unknowns or if
+		// we will use the default Dockerfile.
 		d.Location = filepath.Join(c.Location, "Dockerfile")
 	}