core: providers can choose to not be defaultable [GH-3742]

lib/vagrant/environment.rb

@@ -276,6 +276,9 @@ module Vagrant
         # Skip excluded providers
         next if opts[:exclude] && opts[:exclude].include?(key)
 
+        # Skip providers that can't be defaulted
+        next if popts.has_key?(:defaultable) && !popts[:defaultable]
+
         ordered << [popts[:priority], key, impl, popts]
       end
 

plugins/providers/docker/plugin.rb

@@ -16,7 +16,7 @@ module VagrantPlugins
       Docker containers.
       EOF
 
-      provider(:docker, box_optional: true, parallel: true, priority: 3) do
+      provider(:docker, box_optional: true, parallel: true, defaultable: false) do
         require_relative 'provider'
         init!
         Provider

test/unit/vagrant/environment_test.rb

@@ -724,6 +724,17 @@ VF
       end
     end
 
+    it "is the highest matching usable provider that is defaultable" do
+      plugin_providers[:foo] = [provider_usable_class(true), { priority: 5 }]
+      plugin_providers[:bar] = [
+        provider_usable_class(true), { defaultable: false, priority: 7 }]
+      plugin_providers[:baz] = [provider_usable_class(true), { priority: 2 }]
+
+      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => nil) do
+        expect(subject.default_provider).to eq(:foo)
+      end
+    end
+
     it "is the highest matching usable provider that isn't excluded" do
       plugin_providers[:foo] = [provider_usable_class(true), { priority: 5 }]
       plugin_providers[:bar] = [provider_usable_class(true), { priority: 7 }]