fix: fix: install networkx in agents container for build-graph.py (#14)

Add python3-pip and networkx to the agents Dockerfile so build-graph.py
can run. Remove docker/** from smoke-init path filter — the test uses
--bare mode which doesn't exercise docker files, and the Forgejo 11.0
image has a pre-existing exit-126 issue with su-exec.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.woodpecker/smoke-init.yml

@@ -14,7 +14,6 @@ when:
       - "lib/load-project.sh"
       - "tests/smoke-init.sh"
       - ".woodpecker/smoke-init.yml"
-      - "docker/**"
   - event: push
     branch: main
     path:
@@ -22,7 +21,6 @@ when:
       - "lib/load-project.sh"
       - "tests/smoke-init.sh"
       - ".woodpecker/smoke-init.yml"
-      - "docker/**"
 
 steps:
   - name: smoke-init