Set parallel downloads in Docker to 15

Kubernetes pulls up to 10 images in parallel. If Docker's parallel
downloads is less than this, some downloads might stale and cause
ImagePullBackOff errors.

kubernetes/kubernetes#59376

templates/worker.yaml

@@ -55,6 +55,10 @@ Parameters:
   PrivateSubnet:
     Type: String
 
+  DockerOpts:
+    Type: String
+    Default: "--max-concurrent-downloads=15"
+
   assetBucket:
     Type: String
 
@@ -270,6 +274,13 @@ Resources:
                   }, {
                     "name": "locksmithd.service",
                     "mask": true
+                  }, {
+                    "name": "docker.service",
+                    "enable": true,
+                    "dropins": [{
+                      "name": "20-docker-opts.conf",
+                      "contents": "[Service]\nEnvironment=\"DOCKER_OPTS=${DockerOpts}\""
+                    }]
                   }]
                 },
                 "networkd": {},