adding a walk_dir method to loop over sql script inside sub folders

[hamzawey]

Added a walk_dir method to loop over sql scripts inside sub folders such that

• 001/
  • script001_1.sql
  • script001_2.sql
• 002/
  • script002_1.sql
  • script002_2.sql

[yosifkit]

Duplicate of #192 #193.

Unfortunately this is not something we want to add at this time.

Related issues docker-library/postgres#179, docker-library/postgres#605