Use a specific version of nginx in all tests

There is a hopefully temporary error with nginx:alpine
not being available for amd64. Pick a version which is...

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

projects/ima-namespace/ima-namespace.yml

@@ -17,7 +17,7 @@ services:
   - name: rngd
     image: linuxkit/rngd:v0.2
   - name: nginx
-    image: nginx:alpine
+    image: nginx:1.13.8-alpine
     capabilities:
      - CAP_NET_BIND_SERVICE
      - CAP_CHOWN