* Update alpine from 3.9 to 3.15
This adds support for PostgreSQL 14
* Fix missing python packages
* Install aws-cli as package
* Installing the PostgreSQL client is sufficient
- Bump alpine from 3.9 to 3.11
- Add Postgres v12 support (Fixes#115)
- Add multiple or all databases backup (Fixes#10)
- S3_PREFIX can be empty (Fixes#59)
- Add missing Documentation (Fixes#87)