test: speed up Cucumber stop test (docker#10032)

Evidently `ping` doesn't respond to `SIGTERM`, so use `init` to
get Tini supervising it. This changes the exit code to 143 since
it's not hitting the 10s timeout and getting a `SIGKILL` (137).

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

e2e/cucumber-features/stop.feature

@@ -10,6 +10,7 @@ Background:
           sleep: # will be killed
             image: alpine
             command: ping localhost
+            init: true
         """
 
 Scenario: Cascade stop
@@ -22,7 +23,7 @@ Scenario: Exit code from
     When I run "compose up --exit-code-from sleep"
     Then the output contains "should_fail-1 exited with code 1"
     And the output contains "Aborting on container exit..."
-    And the exit code is 137
+    And the exit code is 143
 
 Scenario: Exit code from unknown service
     When I run "compose up --exit-code-from unknown"