DOCKER: fix a bug in docker building GHA

[jamesbeyond]

Summary of changes

This PR fix a bug in docker building GHA. Which is we should use variable github.repository_owner instead of github.actor

Impact of changes

n/a

Migration actions required

n/a

Documentation

Not Required

---

Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

---

Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

---

Reviewers

---

[ciarmcom]

@jamesbeyond, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started