Enable Docker content trust

[JamieMagee]

This change enabled Docker content trust which verifies the signatures of container images used by docker. This is only done for the base image, which is built on top of Ubuntu.

The signature can be manually verified as follows:

$ docker trust inspect --pretty docker.io/library/ubuntu:22.04

Signatures for docker.io/library/ubuntu:22.04

SIGNED TAG   DIGEST                                                             SIGNERS
22.04        6d7b5d3317a71adb5e175640150e44b8b9a9401a7dd394f44840626aff9fa94d   (Repo Admin)

Administrative keys for docker.io/library/ubuntu:22.04

  Repository Key:       8273733f491f362bb36710fd8a99f78c3fbaecd8d09333985c76f1064b80760f
  Root Key:     1f9bc7ae6335ae41ee03e983c0e31303901be567b4cdb3fc7c7363f0591128ff

Why are we using Docker content trust to verify signatures, when I am talking about signing container images with cosign in #9546 I hear you ask? I hope this explains things: https://xkcd.com/927/