Lock alpine3.14 ( temporary fix for alpine 3.15 newer geos )

[ImreSamu]

upstream docker Postgres - has been upgraded to Alpine 3.15 ( with latest geos )

• Alpine 3.15 docker-library/postgres#910

and now the current code ( 13-3.1/alpine/Dockerfile) is failing

/bin/sh: /usr/lib/llvm12/bin/llvm-lto: not found
make[1]: Leaving directory '/usr/src/postgis/postgis'
make[1]: *** [/usr/local/lib/postgresql/pgxs/src/makefiles/pgxs.mk:236: install] Error 127
make: *** [GNUmakefile:22: install] Error 1

Upgrading Postgis to Alpine 3.15 is still not working ( 2 check fails in Postgis 3.1.4 ) -> #271 (comment)

So this is an alternative quick&dirty fix:

• lock Alpine 3.14
• and wait for the unreleased Postgis 3.1.5

[ImreSamu]

ready to review