Fix export cache issue

Resolve #1551 and #1020.
We are never returning the result of solved operations so Buildkit could not
cache the layer.
This commit implements a simple system to forward operations' result to the
main build to cache it.

Signed-off-by: Vasek - Tom C <tom.chauveau@epitech.eu>

Makefile

@@ -93,4 +93,4 @@ web: # Run the website locally
 
 .PHONY: todo
 todo: # Find all TODO items
-	grep -r -A 1 "TODO:" $(CURDIR)
+	grep -r -A 1 "TODO:" $(CURDIR)