Restrict custom dashboard builds to master branch PRs only

Add base branch check to ensure custom dashboard builds only run
for PRs targeting master. For PRs targeting release branches or
other branches, always use gromit default.

This prevents unnecessary custom builds and ensures release branches
follow their normal workflow.

Related: TT-15971

Author: jeffy-mathew

.github/workflows/release.yml

@@ -435,17 +435,26 @@ jobs:
           echo "📊 Dashboard Image Resolution"
           echo "=================================="
           echo "PR number: $PR_NUMBER"
+          echo "Base ref: $BASE_REF"
           echo "Branch exists: $BRANCH_EXISTS"
           echo "PR image exists: $IMAGE_EXISTS"
           echo "Branch name: $BRANCH"
           echo "PR image tag: $IMAGE_TAG"
-          echo "Base ref: $BASE_REF"
           echo "Commit SHA: $COMMIT_SHA"
           echo "Has relevant changes in latest commit: $HAS_RELEVANT_CHANGES"
           echo "=================================="
 
+          # Only use custom build strategies for PRs targeting master
+          if [ "$BASE_REF" != "master" ]; then
+            echo "ℹ️ Strategy: Use gromit default (base branch is not master)"
+            echo "    → Custom builds only for master branch PRs"
+            echo "dashboard_image=" >> $GITHUB_OUTPUT
+            echo "needs_build=false" >> $GITHUB_OUTPUT
+            echo "dashboard_branch=" >> $GITHUB_OUTPUT
+            echo "strategy=gromit-default" >> $GITHUB_OUTPUT
+
           # Strategy 1: Matching branch exists in tyk-analytics → use gromit
-          if [ "$BRANCH_EXISTS" = "true" ]; then
+          elif [ "$BRANCH_EXISTS" = "true" ]; then
             echo "📋 Strategy: Use matching branch '$BRANCH' via gromit"
             echo "    → No override needed, gromit will handle it"
             echo "dashboard_image=" >> $GITHUB_OUTPUT