ci: skip Docker build workflow in forks

The Docker build workflow requires GHCR_PAT and DOCKERHUB_TOKEN secrets
which are only available in the upstream SignalK/signalk-server repository.
Adding a condition to skip this workflow in forks prevents CI failures.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: mairas

.github/workflows/build-docker.yml

@@ -12,6 +12,8 @@ on:
 
 jobs:
   signalk-server_npm_files:
+    # Only run in the upstream repository (not in forks)
+    if: github.repository == 'SignalK/signalk-server'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout