[DX-3583] Modify legacy nightly system tests triggers

[Tofel]

run legacy nightly system tests with:

1. public images,
2. private images,
3. when nightly image has been built (via docker-build)

Tests:

• public image ✅
• private image ✅
• docker build ❓

[github-actions]

✅ No conflicts with other open PRs targeting develop

[Copilot]

Pull request overview

Risk Rating: MEDIUM — changes affect nightly CI orchestration and image selection for legacy system tests.

Updates legacy nightly system test triggering so runs can be driven via workflow inputs and can also be invoked after the nightly Docker image build completes.

Changes:

• Converted legacy-system-tests-nightly.yml to be driven via workflow_dispatch / workflow_call inputs for Chainlink image selection (full image vs repo+tag) and checkout ref.
• Added Chainlink image resolution + ECR (private/public) login steps to the legacy system tests workflow.
• Updated docker-build.yml to call the legacy nightly system tests workflow on schedule / workflow_dispatch after docker-core finishes, passing the built image tag.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/legacy-system-tests-nightly.yml	Adds inputs and resolves Chainlink image/tag at runtime; updates AWS auth + ECR login to support private/public registries.
.github/workflows/docker-build.yml	Invokes legacy system tests after nightly core image build on scheduled/dispatch runs; aligns chainlink_version passed to downstream workflows with the checked-out ref/SHA.

Scrupulous human review recommended (high-impact blocks):

• .github/workflows/legacy-system-tests-nightly.yml: the Resolve Chainlink image env wiring in both summary and test-nightly jobs (mutual exclusivity handling and correctness of the selected image reference).
• .github/workflows/docker-build.yml: the new call-legacy-system-tests job wiring (trigger conditions + ensuring the intended image/tag is passed through from docker-core outputs).

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.