build: container: Fix call-with-clean-exit.

Before, call-with-clean-exit would *always* return an exit code of 1.

* gnu/build/linux-container.scm (call-with-clean-exit): Exit with status
  code of 0 if thunk does not throw an exception.
* tests/containers.scm: Add test.

gnu/build/linux-container.scm

@@ -36,7 +36,9 @@
   "Apply THUNK, but exit with a status code of 1 if it fails."
   (dynamic-wind
     (const #t)
-    thunk
+    (lambda ()
+      (thunk)
+      (primitive-exit 0))
     (lambda ()
       (primitive-exit 1))))
 

tests/containers.scm

@@ -34,6 +34,10 @@
 
 (test-begin "containers")
 
+(test-assert "call-with-container, exit with 0 when there is no error"
+  (zero?
+   (call-with-container '() (const #t) #:namespaces '(user))))
+
 (test-assert "call-with-container, user namespace"
   (zero?
    (call-with-container '()