Adding PG16 and reworking to minimal Alpine image

[BigAlRender]

We received a ticket from a customer who wanted PG16 backup support. Unfortunately, just adding "16" to the script wasn't cutting it.

Purely as a suggestion, I've created a reworked version based on Alpine.

It makes the image much smaller (~85%):

REPOSITORY              TAG               IMAGE ID       CREATED          SIZE
pg_dump_alpine          latest            8467c6648381   17 minutes ago   178MB
pg_dump_original        latest            ef5ea1b1433d   22 minutes ago   1.15GB

However, it's not all upsides:

• It does require a little more config, ALPINE_VERSION along with POSTGRES_VERSION (values in README)
• Alpine doesn't seem to have an apk package for PG11.

[Weixuanf]

@BigAlRender is this planned to be merged?

[Weixuanf]

if not i guess i'll just fork this as my repo (https://github.com/BigAlRender/postgres-s3-backups/tree/main) and set

POSTGRES_VERSION = 16

ALPINE_VERSION = 3.20

and it should be good to go?