chore: Fix Build Image tags (#272)

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**Description**

Changes proposed in this pull request:

- fixes the provided custom tags

`pull_request_target` makes this PR use the old image build func here.
Didn't test separately again because it has been tested as part of:
kyma-project/lifecycle-manager#1970

**Related issue(s)**
<!-- If you refer to a particular issue, provide its number. For
example, `Resolves #123`, `Fixes #43`, or `See also #33`. -->

- partially closes
https://github.com/orgs/kyma-project/projects/30/views/13?pane=issue&itemId=83441848

.github/workflows/build-image.yml

@@ -1,49 +1,50 @@
-name: build-template-operator
+name: Build Image
 
 on:
   push:
     branches:
-      - main  # This will get tagged with `latest` and `v{{DATE}}-{{COMMIT_HASH_SHORT}}`
+      - main
   workflow_call:
     inputs:
       tag:
-        description: 'Additional tag for built images'
+        description: 'Additional tag for built image'
         required: false
         type: string
         default: ""
   pull_request_target:
     types: [ opened, edited, synchronize, reopened, ready_for_review ]
-
 permissions:
   id-token: write # This is required for requesting the JWT token
   contents: read # This is required for actions/checkout
 
 jobs:
-  compute-tag:
+  get-custom-tags:
     runs-on: ubuntu-latest
     outputs:
-      tag: ${{ steps.get_tag.outputs.tag }}
+      tags: ${{ steps.get_custom_tags.outputs.tags }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-      - name: Get the latest tag
-        id: get_tag
+      - name: Get tags
+        id: get_custom_tags
         run: |
-          if [[ "${{ inputs.tag }}" != "" ]]; then
-            echo "tag=${{ inputs.tag }}" >> $GITHUB_OUTPUT
-          else
-            {
-              echo 'tag<<TAGS'
-              echo 'PR-${{ github.event.pull_request.number }}'
-              echo '${{ github.event.pull_request.head.sha }}'
-              echo 'TAGS'
-            } >> "$GITHUB_OUTPUT"
+          tags=""
+          if [[ "${{ github.event_name }}" == "push" ]]; then
+            tags="latest"
+          elif [[ "${{ github.event_name }}" == "workflow_call" ]]; then
+            tags="${{ inputs.tag }}"
+          elif [[ "${{ github.event_name }}" == "pull_request_target" ]]; then
+            tags="${{ github.event.pull_request.head.sha }}"
           fi
+          echo "Using custom tags: '$tags'"
+          echo "tags=$tags" >> $GITHUB_OUTPUT
   build-image:
-    needs: compute-tag
+    needs: get-custom-tags
     uses: kyma-project/test-infra/.github/workflows/image-builder.yml@main
     with:
       name: template-operator
       dockerfile: Dockerfile
       context: .
-      tags: ${{ needs.compute-tag.outputs.tag }}
+      # tags are additional tags that will be added to the image on top of the default ones
+      # default tags are documented here: https://github.com/kyma-project/test-infra/tree/main/cmd/image-builder#default-tags
+      tags: ${{ needs.get-custom-tags.outputs.tags }}