fix(ci): Serialize Docker builds to avoid runner saturation

Run git-server build after node build completes, and add concurrency
group to cancel duplicate builds on the same ref.

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

Author: ggilestro

.github/workflows/docker.yml

@@ -9,6 +9,10 @@ on:
 env:
   REGISTRY: ghcr.io
 
+concurrency:
+  group: docker-${{ github.ref }}
+  cancel-in-progress: true
+
 permissions:
   contents: read
   packages: write
@@ -50,6 +54,7 @@ jobs:
           cache-to: type=gha,mode=max
 
   build-git-server:
+    needs: build-node
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4