AWS CLI updated to v2; python removed; glibc added
Added support for multiple databases (note POSTGRES_DATABASE env var is deprecated and replaced by POSTGRES_DATABASES) -- this means this version of the docker image is not backward-compatible with previosu releases.
Added support for _FILE env vars for sensitive values. This allows using the image with Docker secrets.
For example,
```
service:
postgres-backup
environment:
POSTGRES_PASSWORD_FILE=/run/secrets/postgres-password
...
secrets:
- postgres-password
```