ci: build head docker images on changes in main

[sydorovdmytro]

Add Docker build step for main branch commits

• Automatically builds a Docker image on every commit to the main branch.
• Tags each Docker image with head-<architecture>, e.g., vcluster-oss:head-arm64v8 or vcluster-oss:head-amd64.

What issue type does this pull request address? (keep at least one, remove the others)
/kind enhancement

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible)
References OPS-148

[Piotr1215]

You can use default branch variable here ${{ github.event.repository.default_branch }}

[Piotr1215]

Left some comments and suggestions. Also we should at least run unit and helm tests before triggering the workflow, something like in the release workflow:

- name: Execute unit tests
  run: ./hack/test.sh

- name: Run Helm Unit Tests
  run: |
    helm plugin install https://github.com/helm-unittest/helm-unittest --version v0.4.4
    helm unittest chart

[sydorovdmytro]

Good idea! Added unit tests and addressed other comments