Reduce the frequency of updates to /build/util/Dockerfile by dependabot

[chengfang]

Is your task related to a problem? Please describe.

dependabot opens PR to update argocd image in /build/util/Dockerfile nearly daily. This is too frequent, too much noise, and wasting ci cycles. See https://github.com/argoproj-labs/argocd-operator/pulls?q=is%3Apr+is%3Aclosed . The latest one as of now is #1049

Describe the solution you'd like

Currently this Dockerfile uses sha256 id as the version number, which may have caused dependabot to detect an update whenever a newer build is published, even if it may be a nightly build.

One solution is to use semver number instead, such as v2.8.3, so that dependabot will open a PR when the next stable version v2.8.4 is available.

One downside is this defeats the purpose of versioning with sha256 for security protection.

Describe alternatives you've considered

some alternatives:

• configure dependabot to ignore updates for argocd in this directory. Instead, we manually update it when needed.
• configure dependabot with less frequent update for argocd in this directory, such as monthly. The monthly PR can serve as a reminder for manual update.

Since there is also a comment of human-readable version (e.g., v2.8.3), so there is always some human editing needed even with dependabot PR, if we keep using sha256 versioning.

Additional context

See update history of this artifact: https://github.com/argoproj-labs/argocd-operator/commits/master/build/util/Dockerfile
argocd images in quay.io: https://quay.io/repository/argoproj/argocd?tab=tags