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>

compose/config.py

@@ -171,6 +171,9 @@ def process_container_options(service_dict, working_dir=None):
     if 'volumes' in service_dict:
         service_dict['volumes'] = resolve_host_paths(service_dict['volumes'], working_dir=working_dir)
 
+    if 'build' in service_dict:
+        service_dict['build'] = resolve_build_path(service_dict['build'], working_dir=working_dir)
+
     return service_dict
 
 
@@ -330,6 +333,17 @@ def resolve_host_path(volume, working_dir):
         return container_path
 
 
+def resolve_build_path(build_path, working_dir=None):
+    if working_dir is None:
+        raise Exception("No working_dir passed to resolve_build_path")
+
+    _path = expand_path(working_dir, build_path)
+    if not os.path.exists(_path) or not os.access(_path, os.R_OK):
+        raise ConfigurationError("build path %s either does not exist or is not accessible." % _path)
+    else:
+        return _path
+
+
 def merge_volumes(base, override):
     d = dict_from_volumes(base)
     d.update(dict_from_volumes(override))