fix: engine/examples/apt-cacher-ng.Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2204-OPENSSL-3043317
- https://snyk.io/vuln/SNYK-UBUNTU2204-OPENSSL-3092568
- https://snyk.io/vuln/SNYK-UBUNTU2204-OPENSSL-3092591
- https://snyk.io/vuln/SNYK-UBUNTU2204-SHADOW-2801886
- https://snyk.io/vuln/SNYK-UBUNTU2204-SHADOW-2801886

engine/examples/apt-cacher-ng.Dockerfile

@@ -5,7 +5,7 @@
 # and then you can run containers with:
 #	docker run -t -i --rm -e http_proxy http://dockerhost:3142/ debian bash
 #
-FROM		ubuntu
+FROM		ubuntu:kinetic-20220830
 
 VOLUME		["/var/cache/apt-cacher-ng"]
 RUN		apt-get update && apt-get install -y apt-cacher-ng