Tell the docker-compose plugin to retry pulls a few times

We see a surprising number of these:

Error response from daemon:
Get "https://973266071021.dkr.ecr.us-east-1.amazonaws.com/v2/":
net/http: request canceled while waiting for connection
(Client.Timeout exceeded while awaiting headers)

lib/buildkite/config/rake_command.rb

@@ -53,6 +53,7 @@ def install_plugins(service = "default", env = nil, dir = ".")
           "env" => env,
           "run" => service,
           "pull" => service,
+          "pull-retries" => 3,
           "config" => ".buildkite/docker-compose.yml",
           "shell" => ["runner", *dir],
         }.compact