Interpret 'build:' as relative to the yml file

* This fix introduces one side-effect: the build parameter is now
validated early, when the service dicionary is first constructed.
That leads to less scary stack traces when the path is not valid.

* The tests for the changes introduced here alter the fixtures
of those (otherwise unrelated) tests that make use of the 'build:'
parameter)

Signed-off-by: Moysés Borges Furtado <moyses.furtado@wplex.com.br>

tests/fixtures/simple-dockerfile/docker-compose.yml

@@ -1,2 +1,2 @@
 simple:
-  build: tests/fixtures/simple-dockerfile
+  build: .