fix: make local Supabase build-from-source work on Alpine bases

[andinux]

Closes #60.

The local build-from-source path (make postgres-supabase-build, docker/postgresql/Dockerfile.supabase) assumed an Ubuntu-userland Supabase base: it installed its toolchain with apt-get and hardcoded the Ubuntu-only pg_config path, so it broke as soon as the Supabase CLI bundled an Alpine-based Postgres image (PG17 17.6.1.084+). #58 fixed the published release image; this is the deferred counterpart.

Approach

Both Supabase base families run a Nix-built, glibc-linked PostgreSQL (verified: the server binary's ELF interpreter is the Nix store's ld-linux, with versioned GLIBC_* symbols — Alpine/musl is only the surrounding userland). So instead of porting apk/apt toolchain detection into the build, the extension is now compiled in a dedicated glibc builder stage:

• Builder: FROM postgres:${PG_MAJOR} (Debian) with build-essential + postgresql-server-dev-${PG_MAJOR} from the preconfigured PGDG repo — the same toolchain family that produces the release artifacts. This yields a glibc .so matching the glibc Postgres on either base, rather than a musl-compiled one that drags a second libc into the server process.
• Runtime: resolves pg_config by probing the Nix profiles then PATH (ported from Dockerfile.supabase.release) and no longer needs a compiler or package manager, so the base userland is irrelevant.
• Makefile: postgres-supabase-build derives PG_MAJOR from the resolved base image tag (default 17), so auto-detection from a running Supabase CLI stack picks matching builder headers for PG15 vs PG17.

Verification

make postgres-supabase-build SUPABASE_POSTGRES_TAG=<tag> followed by CREATE EXTENSION cloudsync; SELECT cloudsync_version(); (as supabase_admin) returns 1.1.2 on:

• 17.6.1.151 (Alpine)
• 17.6.1.071 (Ubuntu, PG17)
• 15.8.1.135 (Ubuntu, PG15 — also exercises the non-17 PG_MAJOR derivation)

The installed cloudsync.so in the Alpine image references versioned GLIBC_* symbols, confirming the matched-libc artifact.

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