fix(docker): use absolute URL for replace-build-vars action ref

Step-level 'uses:' resolves non-fully-qualified action refs against
github.com by default, unlike job-level reusable-workflow 'uses:' which
resolves against the local Gitea instance. Verified via a live CI run
that the shorthand form failed with "authentication required:
Repository not found" trying to clone from github.com/UltimateCoder/...

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

.gitea/workflows/docker.yaml

@@ -44,7 +44,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Replace build placeholders
-        uses: UltimateCoder/resuable-workflows/actions/replace-build-vars@v3
+        uses: https://git.custom-coding.net/UltimateCoder/resuable-workflows/actions/replace-build-vars@v3
 
       - name: Build Image
         run: |