Move docker directory from src/ to top directory.

Currently meta information regarding how to set up build environment
for Mozc and how to build Mozc actually is described in files at the
top level directory (e.g., README.md and appveyor.yml) and documents
under doc/ directory.  One exception of this directory structure is
docker files under src/docker/ directory.

Probably extracting this kind of meta source code out from src/
directory would make sense from the viewpoint of directory structure
management.

doc/build_mozc_in_docker.md

@@ -11,7 +11,7 @@ Currently, only Ubuntu 14.04 is tested to host the Docker container to build Moz
 
 ```
 mkdir ubuntu14.04 && cd ubuntu14.04
-curl -O https://raw.githubusercontent.com/google/mozc/master/src/docker/ubuntu14.04/Dockerfile
+curl -O https://raw.githubusercontent.com/google/mozc/master/docker/ubuntu14.04/Dockerfile
 sudo docker build --rm -t $USER/mozc_ubuntu14.04 .
 sudo docker run --interactive --tty --rm $USER/mozc_ubuntu14.04
 ```
@@ -21,7 +21,7 @@ Fedora 21 container is also provided just for your reference.
 
 ```
 mkdir fedora21 && cd fedora21
-curl -O https://raw.githubusercontent.com/google/mozc/master/src/docker/fedora21/Dockerfile
+curl -O https://raw.githubusercontent.com/google/mozc/master/docker/fedora21/Dockerfile
 sudo docker build --rm -t $USER/mozc_fedora21 .
 sudo docker run --interactive --tty --rm $USER/mozc_fedora21
 ```