fix: engine/examples/postgresql_service.Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2204-GNUPG2-3035409
- https://snyk.io/vuln/SNYK-UBUNTU2204-PERL-2789081
- https://snyk.io/vuln/SNYK-UBUNTU2204-SHADOW-2801886
- https://snyk.io/vuln/SNYK-UBUNTU2204-SHADOW-2801886
- https://snyk.io/vuln/SNYK-UBUNTU2204-ZLIB-2975633

engine/examples/postgresql_service.Dockerfile

@@ -2,7 +2,7 @@
 # example Dockerfile for https://docs.docker.com/examples/postgresql_service/
 #
 
-FROM ubuntu
+FROM ubuntu:22.04
 
 # Add the PostgreSQL PGP key to verify their Debian packages.
 # It should be the same key as https://www.postgresql.org/media/keys/ACCC4CF8.asc