Rename the posgresql to postgres, add the Dockerfile for postgres image.
Build the database image by the variable DB_TYPE, but not mysql.
Add the line to replace the database type.
Replace the 'mysql' options with 'db' options, makeing the config according to the database type.
Replace the 'mysql' options with 'db' options in build.sh
Add the apt source when installing potgres.
Replace the host with $DB_PORT, which is dependent to the database imagae's exposed port.
Merge remote-tracking branch 'origin/master'
Add the support of postgres as the database.
(Config the db in the build.sh file, make the DB_TYPE to 'postgres').