build: add the serve docker tests to the integration tester

These had been disabled on CI for being unreliable, so test them in
the integration tests framework which will retry them.

fstest/test_all/config.yaml

@@ -10,6 +10,8 @@ tests:
  - path: vfs
  - path: cmd/serve/restic
    localonly: true
+ - path: cmd/serve/docker
+   localonly: true
 backends:
  # - backend:  "amazonclouddrive"
  #   remote:   "TestAmazonCloudDrive:"