…b Action.

This work allows to test three types of postgis Docker images to cover
a wider spectrum of geo libraries versions:

* `latest` (recommended upstream): uses latest stable Debian packages.
  These versions are generally conservative, so they may lag behind.
* `alpine`: build PostGIS from source on Alpine, and ship newer
  geospatial libs.
* `master`: provides development versions, therefore coverage for what's
  coming. Future compatibility issues can be caught in advance.

This split is important because each image differs significantly in
GEOS/PROJ/GDAL versions, so testing all increases confidence in
compatibility. More info at https://hub.docker.com/r/postgis/postgis/.

For example, at the time of this branch:

* latest stable in debian:
  * POSTGIS="3.5.2 dea6d0a"
  * GEOS="3.9.0-CAPI-1.16.2"
  * PROJ="7.2.1"
* latest stable in alpine:
  * POSTGIS="3.5.3 0"
  * GEOS="3.13.1-CAPI-1.19.2"
  * PROJ="9.6.0
* latest development branch:
  * POSTGIS="3.6.0dev 3.6.0beta1-29-g7c8cfe07d"
  * GEOS="3.14.0beta2-CAPI-1.20.1"
  * PROJ="9.7.0"

…rts.