test: Set stop_signal to SIGTERM in nginx-based services

The official nginx images set STOPSIGNAL to SIGQUIT which dumps core.
Set it to SIGTERM to avoid dumping core on e2e tests when containers
running "sleep infinity" are stopped.

Signed-off-by: Ricardo Branco <rbranco@suse.de>

pkg/e2e/fixtures/watch/rebuild-deps.yaml

@@ -6,6 +6,7 @@ services:
         RUN echo "backend_build_marker" > /built
     command: sleep infinity
     init: true
+    stop_signal: SIGTERM
   frontend:
     build:
       dockerfile_inline: |
@@ -14,6 +15,7 @@ services:
         RUN echo "frontend_build_marker" > /built
     command: sleep infinity
     init: true
+    stop_signal: SIGTERM
     depends_on:
       - backend
     develop: