Update Postgis docker image to PG12 via custom packages

[MartinMikita]

Hi,

wouldn't be better to use the same process for building openmaptiles/postgis docker image as within GCP, introduced by documentation PR by @nyurik?

this part

Rather than using an older PostgreSQL 11 with PostGIS 2.5, as introduced by PR for v5.0

[nyurik]

Yes and no.

• We should use the base docker image for Postgres & Postgis because of the nuances with how a docker image is built (like setting up start up scripts, etc).
• The process of git clone/compile/cleanup of the additional extensions is very similar, and should be a near copy/paste between the PG docs and the docker image.
• The permissioning may be different (external access to PG server) - so pg_hba.conf won't be the same.
• The Postgres performance optimization (workmem and other settings) should be similar, but may have some differences too because Postgres in docker might not have huge mem pages, etc.

So at the end we will end up with some copy/paste code, but ideally it should be similar with comments linking it together.

Lets keep this ticket open until we get both the docs for GCP and the docker image build process in sync (same versions, same extensions, same process of PG setting optimization)