alpine upgrade to 3.15

[ImreSamu]

• upgrade to alpine 3.15 ( from alpine 3.14 )
• postgis 2.5 : revert geos build and using older package from alpine repo = v3.13
• postgis 2.5 : using older gdal, proj from alpine repo = v3.14
• small refactoring
• added _pgis_full_version.txt debug file for alpine build. ( experimental )

IMHO: the size changes looks normal

# 10-2.5-alpine
postgis/postgis                    10-2.5-alpine      b4b7d6ef936b   33 hours ago        201MB  <-- now: with alpine 3.14
pgis10_25_alpine_v3                latest             cb1fd046db9f   13 minutes ago      198MB  <-- NEW: with alpine 3.15 

# 10-3.2-alpine
postgis/postgis                    10-3.2-alpine      a803a0b7cae5   33 hours ago        208MB  <-- now: with alpine 3.14
pgis10_32_alpine_v3                latest             4306f4c1cb78   10 minutes ago      237MB  <-- NEW: with alpine 3.15  ; newer geos,gdal,proj

the expected new Alpine - postgis versions

• 3.2 POSTGIS="3.2.0 0" [EXTENSION] PGSQL="100" GEOS="3.10.1-CAPI-1.16.0" PROJ="8.2.0" LIBXML="2.9.12" LIBJSON="0.15" LIBPROTOBUF="1.4.0" WAGYU="0.5.0 (Internal)"
• 2.5 POSTGIS="2.5.5" [EXTENSION] PGSQL="100" GEOS="3.8.1-CAPI-1.13.3" PROJ="Rel. 7.2.1, January 1st, 2021" GDAL="GDAL 3.2.3, released 2021/04/27" LIBXML="2.9.12" LIBJSON="0.15" LIBPROTOBUF="1.4.0" RASTER

[ImreSamu]

ready for review ( cc: @phillipross )

[phillipross]

@ImreSamu looks good, thanks! It's ready for merge?

[ImreSamu]

@phillipross: yes; (imho) ready for merge.