Support for backing up all databases

[JJGadgets]

The schickling image supports using pg_dumpall to backup all databases within a Postgres instance/cluster.

https://github.com/schickling/dockerfiles/blob/master/postgres-backup-s3/backup.sh#L65-L88

Could this also be supported in this image? This is the only missing feature holding me back from using this image over schickling's, as I'd like to have the backup be a "set it and forget it" cronjob even as more databases are added.

Would be open to try implementing it myself (will be testing against CloudNative-PG) and PR'ing the changes assuming all goes well.

[eeshugerman]

Interesting, I didn't realize the schickling repo is still merging new code.

Anyway, yes, I would be open to merging a PR for this.

[vutsalsinghal]

@JJGadgets just curious, whats is your reason to want to switch to eeshugerman's image over schickling's? For me its BACKUP_KEEP_DAYS. Ik it can be automated with cron s3 command but since its built-in, its quite convenient!