write non-timestamped tag to GitOps repo

[monotek]

Type of Change

• Bugfix
• Enhancement / new feature
• Refactoring
• Documentation

Description

Branch builds push a timestamped image tag generated with date -u, so
when the action runs across separate invocations (e.g. build and push as
different steps in Staffbase/backend) each invocation recomputes a fresh
timestamp and the tag committed to mops no longer matches the one pushed,
leaving a dangling reference.

Emit a new gitops_tag output (${ALIAS_TAG:-$TAG}) and have the Update
Docker Image step consume it instead of the timestamped tag. The
<prefix>-<short-sha> alias is already pushed alongside and is
deterministic across invocations, so the GitOps reference stays
consistent and resolvable. The image keeps its timestamped tag for Flux
sorting. Works with docker-tag-timestamp true and false; not
configurable.

Checklist

• Write tests
• Make sure all tests pass
• Update documentation
• Review the Contributing Guideline and sign CLA
• Reference relevant issue(s) and close them after merging