fix: use target_tag input for container build when called as subworkflow

Phase 2 built the docker image with tag 'main_build' whenever the
top-level event was a push, but the push step tags it with the
version input ('integration_build'), so the build/push tags diverged
and the push failed. github.event_name reflects the top-level event,
which is 'push' when integration-test triggers main-build from a
branch push. Guard the 'main_build' tag with !is_workflow_call so a
called build uses the provided target_tag, keeping direct pushes to
main unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dorien-er

.github/workflows/main-build.yml

@@ -249,7 +249,7 @@ jobs:
       with:
         config_mod: |
           .version := "${{ inputs.version || format('{0}_build', github.ref_name) }}"
-          .engines[.type == 'docker'].target_tag := '${{ github.event_name == 'push' && 'main_build' || inputs.target_tag }}'
+          .engines[.type == 'docker'].target_tag := '${{ (github.event_name == 'push' && !inputs.is_workflow_call) && 'main_build' || inputs.target_tag }}'
         engine: docker
         src: ${{ matrix.component.dir }}
         setup: build