Update the quickstart guide "docker inspect" section to rightly inspect the image digest for signing

[Roseline-Bassey]

I followed the quickstart guide to sign and validate a container image. Later, I removed the registry using the docker rm -f registry command.

During a second attempt to sign and validate the image, I noticed that the docker inspect localhost:5001/net-monitor:v1 -f '{{ .Id }}' command continued to show the digest value of the previous image: sha256:64e93xxxxxxxda0ab, instead of the newly created image's digest: sha256:cae906b8xxxxxxxx41786

To resolve this issue, @FeynmanZhou suggested using the docker inspect --format='{{index .RepoDigests 0}}' localhost:5001/net-monitor:v1 command. This correctly outputs the digest of the new image.

I will submit a PR with the necessary updates to the quickstart guide for review.