Add replace-build-vars composite action, wire into docker.yaml

Replaces {{BUILD_VAR_*}} placeholders in all text files of the calling
repo's checkout with Gitea Actions build/commit/branch info before the
docker build step runs. Language-agnostic (plain text substitution),
runs unconditionally, no-op when no placeholders are present.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01VVTCJvvJmS4fCSrm5wxwVo

.gitea/workflows/docker.yaml

@@ -43,6 +43,9 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Replace build placeholders
+        uses: UltimateCoder/resuable-workflows/actions/replace-build-vars@v3
+
       - name: Build Image
         run: |
           docker build \