Pinned adobe/s3mock docker image

services/track-changes/docker-compose.ci.yml

@@ -52,7 +52,7 @@ services:
   mongo:
     image: mongo:3.6
   s3:
-    image: adobe/s3mock
+    image: adobe/s3mock:2.1.20
     environment:
       - initialBuckets=fake_user_files,fake_template_files,fake_public_files,bucket
     healthcheck:

services/track-changes/docker-compose.yml

@@ -51,7 +51,7 @@ services:
     image: mongo:3.6
 
   s3:
-    image: adobe/s3mock
+    image: adobe/s3mock:2.1.20
     environment:
       - initialBuckets=fake_user_files,fake_template_files,fake_public_files,bucket
     healthcheck: