ci: use local httpbin container instead of flaky httpbin.org

[shreemaan-abhishek]

Description

The CI route tests proxy through APISIX to the public httpbin.org, which is not very stable. Intermittent 502/503 responses cause spurious result_code: 50x failures on random platforms, requiring re-runs that aren't related to any code change.

This replaces the external dependency with a local httpbin container (the Postman-maintained postmanlabs/httpbin, published as kennethreitz/httpbin and already used in apache/apisix CI), so the route tests resolve httpbin:80 over the compose network instead of reaching out to the internet.

Changes:

• Add an httpbin service to compose/docker-compose-release.yaml and compose/docker-compose-master.yaml on the existing apisix network.
• Point the route upstream in apisix_push_docker_hub.yaml and apisix_dev_push_docker_hub.yaml at httpbin:80 instead of httpbin.org:80.

Verification

docker compose config validates both compose files. The kennethreitz/httpbin container serves GET /get with a 200, matching the route the tests exercise.