Add merge_group trigger and fix Docker caching in CI workflows

- PR-Build-And-Test.yml: Add merge_group trigger so Docker validation
  and tests run in the GitHub merge queue
- PR-Build-And-Test.yml: Replace outputs: type=docker,dest=*.tar with
  push: false — tar was never uploaded as artifact; this skips the
  slow layer extraction while still populating the GHA cache
- Build-Test-And-Deploy.yml: Remove dead "Docker build (no push)" step
  whose condition (pull_request || merge_group) could never be true
  since the workflow only triggers on push to main and workflow_dispatch;
  PR/merge_group validation is now handled by PR-Build-And-Test.yml

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: BenjaminMichaelis

.github/workflows/Build-Test-And-Deploy.yml

@@ -63,17 +63,6 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
 
-      # Build but no push with a PR
-      - name: Docker build (no push)
-        if: github.event_name == 'pull_request' || github.event_name == 'merge_group'
-        uses: docker/build-push-action@v7
-        with:
-          push: false
-          tags: temp-pr-validation
-          file: ./EssentialCSharp.Web/Dockerfile
-          context: .
-          build-args: ACCESS_TO_NUGET_FEED=false
-
       # Only build for dev registry — prod gets the image via az acr import in deploy-production
       - name: Build Container Image
         if: github.event_name != 'pull_request_target' && github.event_name != 'pull_request'

.github/workflows/PR-Build-And-Test.yml

@@ -3,6 +3,7 @@ name: PR Build and Test EssentialCSharp.Web
 on:
   pull_request:
     branches: ["main"]
+  merge_group:
   workflow_dispatch:
 
 jobs:
@@ -67,7 +68,7 @@ jobs:
         with:
           file: ./EssentialCSharp.Web/Dockerfile
           context: .
-          outputs: type=docker,dest=${{ github.workspace }}/essentialcsharpwebimage.tar
+          push: false
           cache-from: type=gha
           cache-to: type=gha,mode=max
           build-args: ACCESS_TO_NUGET_FEED=false