manually build gosu with current golang image to resolve CVE (pull/1323)

https://github.com/docker-library/postgres/pull/1323

trivy image --scanners vuln
shows that we'll get rid of the following CVE's

usr/local/bin/gosu (gobinary)

Total: 58 (UNKNOWN: 0, LOW: 1, MEDIUM: 23, HIGH: 31, CRITICAL: 3)

gosu security policy https://github.com/tianon/gosu/blob/master/SECURITY.md says they don't update golang for CVE's
So gosu is build with a unsupported version of go (1.20)
The two support go versions that have the most CVE's resolved are 1.23.6 and 1.24.0
I felt using 1.23.6 was a safer upgrade.

Your own documents talk about gosu, and I believe gosu hase a fundamental misunderstanding about what a vulnerability free govulncheck means. I understand it to mean that you don't have any external dependencies with vulnerabilities, not that you shouldn't update compiler versions. They are 2 different things.

This PR removes the CVE caused by gosu by doing a custom build of gosu with a currently supported go version.
And copies it into the final image.

Thanks for your consideration

Start by reviewing pull/1323 and the Postgres image build entry points that produce usr/local/bin/gosu. Run trivy image --scanners vuln against the resulting image and compare its findings with the reported CVEs. Done means the image uses a gosu binary built with a supported Go version and the reported vulnerabilities are resolved.