Add Debian docker capability for Docker's official packege

/lib/systemd/system/docker.service in Docker's officaial package
does not include /etc/default/docker.

lib/vagrant-proxyconf/cap/debian/docker_proxy_conf.rb

@@ -0,0 +1,32 @@
+require_relative '../util'
+
+module VagrantPlugins
+  module ProxyConf
+    module Cap
+      module Debian
+        # Capability for docker proxy configuration
+        module DockerProxyConf
+          # @return [String, false] the path to docker or `false` if not found
+          def self.docker_proxy_conf(machine)
+            return false unless Util.which(machine, 'docker')
+
+            machine.communicate.tap do |comm|
+              src_file = '/lib/systemd/system/docker.service'
+              dst_file = '/etc/systemd/system/docker.service'
+              tmp_file = '/tmp/docker.service'
+              env_file = 'EnvironmentFile=-\/etc\/default\/docker'
+              comm.sudo("sed -e 's/\\[Service\\]/[Service]\\n#{env_file}/g' #{src_file} > #{tmp_file}")
+              unless comm.test("diff #{tmp_file} #{dst_file}")
+                # update config and restart docker when config changed
+                comm.sudo("mv -f #{tmp_file} #{dst_file}")
+                comm.sudo('systemctl daemon-reload')
+              end
+              comm.sudo("rm -f #{tmp_file}")
+            end
+            '/etc/default/docker'
+          end
+        end
+      end
+    end
+  end
+end

lib/vagrant-proxyconf/capability.rb

@@ -18,6 +18,11 @@ class Plugin < Vagrant.plugin('2')
         Cap::CoreOS::DockerProxyConf
       end
 
+      guest_capability 'debian', 'docker_proxy_conf' do
+        require_relative 'cap/debian/docker_proxy_conf'
+        Cap::Debian::DockerProxyConf
+      end
+
       guest_capability 'linux', 'env_proxy_conf' do
         require_relative 'cap/linux/env_proxy_conf'
         Cap::Linux::EnvProxyConf