ci: add path filters to Docker build workflow

Only build Docker images when backend/, frontend/, docker-compose,
or the workflow itself changes. Prevents unnecessary image builds
for docs-only or config-only changes on main.

Note: paths filter is not evaluated for tag pushes (GitHub Actions
behavior), so release tags always trigger a full build.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/docker-build.yml

@@ -4,6 +4,12 @@ on:
   push:
     branches: [main]
     tags: ['v*']
+    paths:
+      - 'backend/**'
+      - 'frontend/**'
+      - 'docker-compose.yml'
+      - 'docker-compose.dev.yml'
+      - '.github/workflows/docker-build.yml'
   workflow_dispatch:
     inputs:
       tag: