ci: fix Docker GHA caching and add merge_group trigger

[BenjaminMichaelis]

Summary

Docker GHA caching (type=gha) was already in both CI workflows, but three issues made it less effective than it should be.

Changes

PR-Build-And-Test.yml

Add merge_group trigger

Docker validation and tests were not running in the GitHub merge queue, meaning a merge could pass without a final Docker build check. Adding merge_group: ensures the same validation that runs on PRs also runs before merge.

Replace tar export with push: false

The Docker step was exporting the image to a .tar file (outputs: type=docker,dest=...) but that file was never uploaded as an artifact and never used by any downstream job. Exporting to a tar forces BuildKit to fully extract all image layers to disk — adding unnecessary time and disk I/O. Replacing it with push: false skips the extraction while still writing all layers to the GHA cache.

Build-Test-And-Deploy.yml

Remove dead "Docker build (no push)" step

The step had this condition:

if: github.event_name == 'pull_request' || github.event_name == 'merge_group'

But this workflow only triggers on push: branches: [main] and workflow_dispatch — so that condition can never be true. The step was dead code. PR and merge_group Docker validation is now correctly owned by PR-Build-And-Test.yml (via the merge_group trigger added above).

Before / After

Workflow	Before	After
PR Docker build	Exports unused tar, no merge queue coverage	push: false, runs on merge queue too
Main Docker build	Has unreachable dead step	Dead step removed; caching unchanged

[Copilot]

Pull request overview

This PR improves CI Docker validation by making PR builds compatible with merge queue checks and avoiding unnecessary Docker image tar export in PR validation.

Changes:

• Adds merge_group to the PR build/test workflow triggers.
• Changes PR Docker validation from unused tar output to push: false.
• Removes an unreachable Docker validation step from the deployment workflow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/PR-Build-And-Test.yml	Adds merge queue coverage and simplifies Docker validation output behavior.
.github/workflows/Build-Test-And-Deploy.yml	Removes dead PR/merge-group Docker build logic from a workflow that only runs on push/manual dispatch.