fix(#363): rebuild gosu from source with Go 1.26 to fix CRITICAL CVEs

The gosu 1.19 binary bundled in the postgres base image was compiled
with Go 1.24.6, which contains CVE-2025-68121 (CRITICAL) and 5 HIGH
severity Go stdlib vulnerabilities. Since upstream gosu has not released
a version built with patched Go (1.24.13+ / 1.25.7+), this adds a
multi-stage Docker build that recompiles gosu from source using Go 1.26.

Changes:
- Pin postgres base image to 17.7-alpine3.22 for reproducibility
- Add golang:1.26-alpine3.22 builder stage to compile gosu v1.19
- Replace bundled gosu binary with freshly built version
- Pin all postgres:17-alpine references across compose files and CI

CVEs fixed:
- CVE-2025-68121 (CRITICAL): Go crypto/tls vulnerability
- CVE-2025-58183 (HIGH): Go archive/tar unbounded allocation
- CVE-2025-61726 (HIGH): Go net/url memory exhaustion
- CVE-2025-61728 (HIGH): Go archive/zip CPU exhaustion
- CVE-2025-61729 (HIGH): Go crypto/x509 DoS
- CVE-2025-61730 (HIGH): Go TLS 1.3 handshake vulnerability

Fixes #363

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/docker-compose.build.yml

@@ -71,7 +71,7 @@ services:
   # Authentik PostgreSQL
   # ======================
   authentik-postgres:
-    image: postgres:17-alpine
+    image: postgres:17.7-alpine3.22
     container_name: mosaic-authentik-postgres
     restart: unless-stopped
     environment: