Modify `.kitchen.yml` to support `kitchen-docker`.

```
diff --git a/site-cookbooks/fluentd-custom/.kitchen.yml b/site-cookbooks/fluentd-custom/.kitchen.yml
index f27bf87..14268d2 100644
--- a/site-cookbooks/fluentd-custom/.kitchen.yml
+++ b/site-cookbooks/fluentd-custom/.kitchen.yml
@@ -1,6 +1,7 @@
 ---
 driver:
-  name: vagrant
+  name: docker
+  socket: <%= ENV['DOCKER_HOST'] %>

 provisioner:
   name: chef_solo
@@ -8,14 +9,13 @@ provisioner:
   data_bags_path: ../../data_bags
   roles_path: ../../roles

-
 platforms:
   - name: ubuntu-12.04
-    driver:
-      box: ubuntu-1204x64
+    driver_config:
+      image: kazu634:ubuntu1204
   - name: ubuntu-14.04
-    driver:
-      box: ubuntu-1404x64
+    driver_config:
+      image: kazu634:ubuntu1404

 suites:
   - name: default
```
Install `build-essentials` when building `Docker` image.

```
diff --git a/docker/ubuntu1204/Dockerfile b/docker/ubuntu1204/Dockerfile
index 2814d73..ef76bd4 100644
--- a/docker/ubuntu1204/Dockerfile
+++ b/docker/ubuntu1204/Dockerfile
@@ -4,5 +4,6 @@ RUN sed -i "s/archive.ubuntu.com/jp.archive.ubuntu.com/" /etc/apt/sources.list

 RUN apt-get update
 RUN apt-get install -y sudo openssh-server curl lsb-release
+RUN apt-get install -y build-essential

 RUN curl -L https://www.opscode.com/chef/install.sh | bash
```

```
diff --git a/docker/ubuntu1404/Dockerfile b/docker/ubuntu1404/Dockerfile
index dbeae19..03ec2fb 100644
--- a/docker/ubuntu1404/Dockerfile
+++ b/docker/ubuntu1404/Dockerfile
@@ -4,5 +4,6 @@ RUN sed -i "s/archive.ubuntu.com/jp.archive.ubuntu.com/" /etc/apt/sources.list

 RUN apt-get update
 RUN apt-get install -y sudo openssh-server curl lsb-release
+RUN apt-get install -y build-essential

 RUN curl -L https://www.opscode.com/chef/install.sh | bash
```