DEVEX-86 Fix Docker blocking on start

Startup process was hanging forever and it looked like Docker was responsible. We dug into
the Docker API gem which looked like it was using Excon. It looked like it was using a
blocking connection, so we pass 'nonblock' option to unblock it.

https://github.com/excon/excon/search\?utf8\=%E2%9C%93\&q\=nonblock\&type\=
https://spin.atomicobject.com/2013/09/30/socket-connection-timeout-ruby/
http://www.mikeperham.com/2009/03/15/socket-timeouts-in-ruby/

upserve/docker-api#515

config/initializers/docker.rb

@@ -9,7 +9,11 @@
     Docker.url = url
   end
 
-  Docker.options = { read_timeout: Integer(ENV['DOCKER_READ_TIMEOUT'] || '10'), connect_timeout: 2 }
+  Docker.options = {
+    read_timeout: Integer(ENV['DOCKER_READ_TIMEOUT'] || '10'),
+    connect_timeout: 2,
+    nonblock: true
+  }
 
   begin
     Docker.validate_version! # Confirm the Docker daemon is a recent enough version
@@ -21,12 +25,12 @@
   # ensure that --cache-from is supported (v13+)
   min_version = 13
   begin
-    Timeout.timeout(1) do
-      local = Integer(`docker -v`[/Docker version (\d+)/, 1])
-      server = Integer(Docker.version.fetch("Version")[/\d+/, 0])
-      if local < min_version || server < min_version
-        raise Docker::Error::VersionError, "Expected docker version to be >= #{min_version}"
-      end
+    local = Timeout.timeout(1) do
+      Integer(`docker -v`[/Docker version (\d+)/, 1])
+    end
+    server = Integer(Docker.version.fetch("Version")[/\d+/, 0])
+    if local < min_version || server < min_version
+      raise Docker::Error::VersionError, "Expected docker version to be >= #{min_version}"
     end
   rescue
     warn "Unable to verify local docker!"