Dockerfile 에서 SECRECT_KEY_BASE 제거
mysql 사용을 위한 Dockerfile 수정
mysql2 gem 추가
Production DB 설정
Production DB 비밀번호 환경변수 변경
docker rake task 수정

- mysql docker 사용을 위한 설정으로 변경
README Docker 설명 변경
Merge pull request #17 from seapy/master

운영환경에서 MySQL 사용