Add Dockerfile for OSS Scanner

On 2026-10-08, Anthropic launched its "OSS Scanner"
(https://www.anthropic.com/research/launching-opt-in-vuln-finding-service-for-open-source)
The plan is to enroll security(at)postgresql(dot)org for automated notification of
vulnerabilities it finds. One of the steps is to provide a Dockerfile that
the scanner will use to get a PostgreSQL build. The attached patch adds one
to postgresql.git. It mimics CI.

If using the test recipe in the header comment, change "git clone
https://github.com/anthropics/oss-scanner/" to "git clone -b enroll-postgresql
https://github.com/nmisch/oss-scanner/". That's a temporary need until
enrollment completes.