providers/docker: Fix usability check

In commit 7980178d19 (#10879) I added a
`usable?` class method to `VagrantPlugins::DockerProvider::Provider`.
However, commit 34e53a5a4b (#10890)
incorrectly changed it to an instance method.  This rendered it
ineffective because it’s called on the class, not an instance.  Change
it back to a class method.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

plugins/providers/docker/provider.rb

@@ -11,6 +11,14 @@ module VagrantPlugins
     class Provider < Vagrant.plugin("2", :provider)
       @@host_vm_mutex = Mutex.new
 
+      def self.usable?(raise_error=false)
+        Driver.new.execute("docker", "version")
+        true
+      rescue Vagrant::Errors::CommandUnavailable, Errors::ExecuteError
+        raise if raise_error
+        return false
+      end
+
       def initialize(machine)
         @logger  = Log4r::Logger.new("vagrant::provider::docker")
         @machine = machine
@@ -45,14 +53,6 @@ module VagrantPlugins
         @driver
       end
 
-      def usable?(raise_error=false)
-        driver.execute("docker", "version")
-        true
-      rescue Vagrant::Errors::CommandUnavailable, Errors::ExecuteError
-        raise if raise_error
-        return false
-      end
-
       # This returns the {Vagrant::Machine} that is our host machine.
       # It does not perform any action on the machine or verify it is
       # running.

test/unit/plugins/providers/docker/action/connect_networks_test.rb

@@ -4,7 +4,6 @@ require_relative "../../../../../../plugins/providers/docker/action/connect_netw
 
 describe VagrantPlugins::DockerProvider::Action::ConnectNetworks do
   include_context "unit"
-  include_context "virtualbox"
 
   let(:sandbox) { isolated_environment }
 

test/unit/plugins/providers/docker/action/login_test.rb

@@ -4,7 +4,6 @@ require_relative "../../../../../../plugins/providers/docker/action/login"
 
 describe VagrantPlugins::DockerProvider::Action::Login do
   include_context "unit"
-  include_context "virtualbox"
 
   let(:sandbox) { isolated_environment }
 

test/unit/plugins/providers/docker/action/prepare_networks_test.rb

@@ -3,7 +3,6 @@ require_relative "../../../../../../plugins/providers/docker/action/prepare_netw
 
 describe VagrantPlugins::DockerProvider::Action::PrepareNetworks do
   include_context "unit"
-  include_context "virtualbox"
 
   let(:sandbox) { isolated_environment }
 

test/unit/plugins/providers/docker/provider_test.rb

@@ -21,9 +21,12 @@ describe VagrantPlugins::DockerProvider::Provider do
   end
 
   describe ".usable?" do
+    subject { described_class }
+
     it "returns true if usable" do
+      allow(VagrantPlugins::DockerProvider::Driver).to receive(:new).and_return(driver_obj)
       allow(provider_config).to receive(:compose).and_return(false)
-      allow(subject.driver).to receive(:execute).with("docker", "version").and_return(true)
+      allow(driver_obj).to receive(:execute).with("docker", "version").and_return(true)
       expect(subject).to be_usable
     end