ci(docker): set GHA cache scope to main for cross-tag cache hits

Tags resolve to different scope keys, causing cache misses on every
release build. Hardcoding to main ensures the Docker layer cache is
shared across all releases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kgritesh

.github/workflows/release.yml

@@ -41,8 +41,8 @@ jobs:
           tags: |
             vertexcoverlabs/linkedin-mcp:${{ steps.tag_version.outputs.VERSION }}
             vertexcoverlabs/linkedin-mcp:latest
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-from: type=gha,scope=main
+          cache-to: type=gha,mode=max,scope=main
 
   release:
     runs-on: ubuntu-latest