fix: merge general-use steps properly

[rangoo94]

Pull request description

• It was incorrectly using default image when the container had pure operation merged forward

kind: TestWorkflow
apiVersion: testworkflows.testkube.io/v1
metadata:
  name: example
spec:
  steps:
  - delay: 1s
    run:
      image: curlimages/curl:latest
      shell: curl -XGET http://testkube.io/

It will run both commands with the default init image, instead of curlimages/curl:latest

Checklist (choose whats happened)

• breaking change! (describe)
• tested locally
• tested on cluster
• added new dependencies
• updated the docs
• added a test