Readme updates

[chrisns]

Summary

Hi,

There's a few things going on here, I'd be very happy to split into seperate PRs if you'd prefer please do let me know.

If you do approve I'd really appreciate if you could add the hacktoberfest label to this PR (or the repo).

Many thanks!

• It appeared from the workflow's in the readme that the key was in the repo and no passphrase was used, which unless I'm missing something seems like a bad idea, so I've moved these to GitHub secrets.
• I removed a few redundant params from the docker build step
• I've used the docker_meta action to keep the image name and tags in one place rather than repeat it (DRY)
• I've used the GitHub OIDC with the COSIGN_EXPERIMENTAL flag I did some proof of concept work on this here https://github.com/chrisns/cosign-keyless-demo

Release Note

Documentation updates to describe using GitHub OIDC and keeping key material in GitHub Secrets

[cpanato]

cool thanks!