Image must not be tagged as `SNAPSHOT` when it was not build from `main`

[kthoms]

Images are currently tagged as SNAPSHOT when they are build for snapshot versions. But there are multiple streams that trigger Docker image builds

• main
• release/1.0.x
• release/2.0.x
  (any release branch)

The official "SNAPSHOT" should be the latest development version, which is the one built from main on the operaton/operaton repository.

Expected result

The tag SNAPSHOT is not created for builds from release branches.