Add signon-app to Whitehall/Mainstream

Whitehall (and Mainstream shortly) will soon need access to Signon to
fetch user information about updated content blocks (see alphagov/publisher#2513
and alphagov/whitehall#9754)

Author: pezholio

projects/publisher/docker-compose.yml

@@ -38,6 +38,7 @@ services:
       - link-checker-api-app
       - publisher-worker
       - publisher-css
+      - signon-app
     environment:
       MONGODB_URI: "mongodb://mongo-3.6/publisher"
       REDIS_URL: redis://publisher-redis
@@ -64,4 +65,4 @@ services:
     command: bin/dev worker
 
   publisher-redis:
-    image: redis
+    image: redis

projects/whitehall/docker-compose.yml

@@ -42,6 +42,7 @@ services:
       - asset-manager-app
       - publishing-api-app
       - whitehall-worker
+      - signon-app
     environment:
       GOVUK_PROXY_STATIC_ENABLED: "true"
       GOVUK_ASSET_ROOT: "http://asset-manager.dev.gov.uk"
@@ -68,4 +69,4 @@ services:
     command: bundle exec sidekiq -C ./config/sidekiq.yml
 
   whitehall-redis:
-    image: redis
+    image: redis